A single photon avalanche diode designed with 0.18um CMOS technology is proposed, with which the weak light in 650~950nm wavelength can effectively detected. The single photon avalanche diode employs a P+/N-well structure, the deeper P+ layer is designed to improve the photon detection efficiency and responsivity in longer light wave; moreover, the increase of the thickness of depletion layer can be got with the use of the low doped deep N well, which can improve the detection sensitivity, and the PN junction formed by deep N well and substrate can effectively isolate substrate in order to reduce substrate noise. The use of P-well as the protection ring can prevent premature edge breakdown phenomenon. The basic structural parameters and process parameters of single photon avalanche diode devices are determined by theoretical analysis, and the device performance is optimized with Silvaco TCAD software. The simulation results show that when the optical window diameter of the device is 10 μm, the avalanche breakdown voltage is 18.4 V, with the illumination of 0.001 W/cm2, the peak of response is of 0.495 A/W at 650 nm will be got. when the reverse bias is excessed the breakdown voltage for 2 V, in the range of 650~950 nm wavelength, the photon detection efficiency is higher than 30%. With the increase of reverse bias, the detection efficiency is improved.