The high sensitivity APD arrays have more and more application in the data transmission, LIDAR, remote sensing, medical image diagnosis system, environmental monitoring, military reconnaissance and etc. A preliminary study of Si APD was carried out, including the simulation of the photoelectric characteristics of Si APD, the experiment of Si APD single chip and array, and the test of Si APD. The APD gain is above 100, dark current is several nA, the rise time is nanosecond level. The 4×4, 1×16 Si APD arrays with high gain, quick response and low dark current have been made by means of available conventional semiconductor technology. The pulse width of the transient response under 1064 nm pulse LD illuminated is less than 100 ns at 100 V bias voltage which the pulse width is limited by the following amplification circuit. Some measures to improve the responsivity of APD at 1064nm is discussed. The next step is to develop the CMOS compatible high sensitivity APD array integrated with CMOS readout circuit.