To measure small particles in clouds without the optical amplification system,a new type of p-i-n photodetector linear array with 128 diode units altogether is designed and realized.In each die,there are two rows of photodiode line array,and each row has 64 photodiodes.Every photodiode has a size of 100 μm × 100 μm with an individual output,and each of them is isolated by the trenches.The depth of them has the same thickness as that of the epitaxial layer,which is designed to be 30 μm to guarantee sufficient absorption of photons and leave a margin for the diffusion of p-type and n-type region.The detector has been tested with a laser whose wavelength was 650nm and irradiance is 50mW/cm2.The achieved photocurrent is 2μA.Hence,the current responsivity is about 0.4A/W,and the exteal quantum efficiency is 76.45％.The dark current is less than 600pA.Both of the sufficient absorption of photons and low dark current are achieved by utilizing the thick epitaxial intrinsic layer.Low interference of adjacent photodiodes is also guaranteed by the trenches around the photodiodes.With the obtained performance,the photodetector can be used to measure the diameter of precipitation particles in clouds.Therefore,rainfall can be judged based on the diameter of particles.