Reading ability is one of key components for second language learners to study a second language. With the respect of the importance of reading ability， the crucial question is the best way to improve second language learner’s （L2） reading ability. In the past decades， a lot of research on extensive reading has been carried out to explore its effect on second language learners reading ability. In the past decades， the original definition of extensive reading is that reading a lot of materials， reading fast， and a lot of reading experiences， and these aspects are applied in early extensive reading. After that and with more and more researchers pay attention to improving second language learners’ reading ability， extensive reading becomes one of the controversial instructions for second language learners. With the respect on researches which have been done， reading ability is likely to be enhanced when L2 has gotten great deal of reading input through extensive reading. “Extensive reading has been proposed as one of the most effective methods for developing reading abilities because it can expose L2 learners to ample amounts of meaningful input， motivate L2 learners to read， and lead to the development of skilled reading.” （Grabe， 2009， p. 375）
　　It’s important to know that for extensive reading which is affected a lot by which way it is perceived. Grabe defined extensive reading as an approach “in which learners read large quantities of materials that are within their linguistic competence”（Grabe， 2011， p. 27）. And Bamford also stated that， “extensive reading is an approach to language teaching in which learners read a lot of easy materials in the new language” （Grabe， 2009， p. 377）. From these two descriptions of extensive reading which we can know that learners will read a lot of reading materials and gain a great deal of reading exposure from extensive reading， and learners need to read the reading materials which can be suitable for their English levels to gain great deal of exposure from extensive reading. For more specific definition of extensive reading， there are three aspects of it. One is that reading materials should be suitable for L2’s language levels which means reading materials must be easy enough for them to comprehend without burden. Bamford （1998） stressed that “reading within the reader’s linguistic competence can contribute to developing a sight vocabulary and thus lead to reading fluency. “（Bamford， 1998， p. 105） The second is that extensive reading requires great deal of meaningful exposure in the second language， which is the key for second language learning. “By being exposed to large quantities of meaningful reading materials for an extended period of time， reading fluency and reading comprehension can develop incrementally”（Grabe， 2009， p. 350）. The third is that extensive reading provides readers opportunities to select reading materials which they are interested in， which will enhance L2 learners’ motivation to read. These three aspects of extensive reading require L2 to reading a lot of reading materials which they are interested in and which are suitable for their language levels， leading second language development. 　　In the past decades， a lot of researches have been conducted and most of them show that extensive reading can improve L2’s reading ability with positive results in the areas of vocabulary development， and improving reading fluency and reading comprehension. In this paper， my statement is that extensive reading has the positive effect on second language learners’ reading ability through improving their vocabulary development， reading rates and reading comprehension.
　　Studies have been done to explore whether extensive reading has positive effect on vocabulary development. Studies have identified positive effects of extensive reading on vocabulary acquisition. Extensive reading can enhance L2’s vocabulary development through vocabulary acquisition. Pigada and Schmitt （2006） stated that second language learner’s lexical knowledge can be enhanced through extensive reading. （Pigada