Two-dimensional (2-D) photo-count mapping on CMOS single photon avalanche diodes (SPADs) has been demonstrated. Together with the varied incident wavelengths, the depth-dependent electric field distribution in active region has been investigated on two SPADs with different structures. Clear but different non-uniformity of photo-response have been observed for the two studied devices. With the help of simulation tool, the non-uniform photo-counts arising from the electric field nonuniformity have been well explained. As the quasi-3D distribution of electric field in the active region can be mapped, our method is useful for engineering the device structure to improve the photo-response of SPADs.