In order to further diminish the size of SPAD detector, the guard ring size of p-well/DNW (deep n-well) SPAD was designed based on a 0.18 μm CMOS Image Sensor (CIS) technology and SPADs of varied guard ring sizes were fabricated. The measured results show that, the guard ring with its size decreased to 0.4 μm is still effective in preventing premature edge breakdown (PEB), and the guard ring size does not have a significant impact on the dark count rate (DCR) and the photon detection probability (PDP) of p-well/DNW SPADs. The SPADs achieve a low DCR and a broad spectral response, and the DCR is 638 Hz at 25℃ with a PDP peak of 16% at 530 nm for 20 μm diameter active area structure.