As a media service platform,The video broadcast system which integrates current advanced video processing technology and network transmission technology has become an indispensable and important part of subway multimedia system.With the mature application of fast Ethernet and gigabit network in the subway network system,it has already become a mainstream to achieve high-definition and intelligent video screen in the subway on-board.Although the domestic subway on-board video broadcast system has succeed in playing high-definition video at present,there are still some problems such as video processing terminal crashes,low image clarity and poor playback,which seriously affect the passenger’s journey experience.To cope with the above problems,this paper designed a high-definition video broadcast system on the basis of subway on-board condition to achieve the smooth playback of high-definition video.The main contents of this paper are as follows:1.Due to the low encoding efficiency and high computational complexity of H.264 encoding standard,an optimization scheme of H₂64 intra prediction algorithm and encoder option is developed.A H.264 the fast prediction algorithm based on the combination of SATD and the image itself,the H.264 encoding options for the subway network frame is proposed.The result of experiment show that the coding time of the algorithm is shortened by 50%on average after optimization and the efficient encoding of the video is realized on the server side under the condition of the quality of the video picture.2.Because of packet loss and wrong packet transmission at the current in the process of video transmission UDP in subway on-board network,an optimization mechanism in the application layer based on reliable UDP is suggested.The results of experiments show that packet loss rate decreased by an average of 62.33%in the sudden change of the subway network environment based on the application layer UDP video data transmission.3.Design the high-definition video broadcast platform based on "Quanzhi A20 dual-core processor and Linux operating system".The platform server adopts the optimized H.264 encoding scheme to realize the video coding.Meanwhile,the video transmission is achieved through the reliable UDP based on the application layer and the client uses FFmpeg to decode and play the high-definition video.Experiments show that the similarity between decoded and uncompressed video is about 0.98,the average peak of signal noise ratio is about 33dB and the average frame rate is above 27fps.The system has a low packet loss rate and achieves a smooth video showing.This paper designs a high-definition video broadcast system in the subway to realize the smooth playback of high-definition video in the subway on-board environment on the basis of the optimization of the server video coding scheme and network transmission protocol.The system is not only suitable for subway on-board,but also has good applicability in the bus carrying system and customer service hall.And it will have broad application prospects.