The skin is the largest organ in the human body, and it is an important window for optical imaging and diagnosis. However, the turbid characteristics of skin limits the penetration of light. Skin optical clearing technique provides an important tool for improving optical imaging performance. However, study on mechanism for skin optical clearing was mostly based on ex vivo skin experiments, the in vivo skin optical clearing process still needs further exploration. In this work, we can in vivo monitor the changes in skin during treatment of skin optical clearing agent by using two-photon imaging. For untreated skin, collagen has a strong second harmonic generation (SHG) signal, the structure of epidermis and collagen is relatively complete. After treatment of skin optical clearing agent, the SHG signal of collagen is significantly weakened. After optical clearing agent removed and saline treated, the skin can recover to initial state obviously, and the SHG is greatly enhanced, even is stronger than initial level. The results indicated that the treatment of optical clearing agents (OCA) can induce the structural change of skin, which lead to skin transparent consequently, and optical clearing agent–induced decrease of collagen SHG is invertible by recovery with treatment of physiological saline. This work is very helpful for understanding the in vivo skin optical clearing process, which is of great significant to revealing the mechanism of in vivo skin optical clearing.