Intensity of laser focus was simulated in the condition of refractive index mismatch (n=1.48, NA=0.65) at different recording depth with two-photon writing based on Torok’s theory,in order to increase the readout signal intensity of deep layer bits and the memory capacity of two-photon 3D optical data storage. The relationship between the square of the maximum intensity at laser focus and the recording depth was acquired. Based on atomic absorption theory, the relationship between the exposure time and the recording depth in the condition of equal exposure energy without recording power changed was acquired. According to the results and using home-made two-photon 3D optical data memory system, the contrast experiment of recorded bits in eight consecutive layers with equal exposure energy and excitation time increasing was done in a photochromic storage medium. The experimental result indicates that the memory capacity is increased and the readout signal intensity in deep layer is enhanced effectively.