The storage mechanism of photochromic three-dimensional optical storage under two-photon absorption is presented. The experiments on the effect of writing power, exposure time and recording depth on the contrast of the readout signal intensity and bit size were carried out by use of three-dimensional optical memory setup with single-beam two-photon and a reflection confocal scanning readout setup. The results show that the signal intensity and the bit size of the recorded bits depend strongly on the writing power and exposure time. The signal intensity initially shows an I2 dependence on power, followed by saturation. The saturation effect is also evident as exposure time is longer. Bit size is increased with writing power or exposure time increased. Furthermore, signal intensity is decreased with an increase in recording depth. Storage rate is higher by use of higher power and shorter exposure time.