Multi-level run-length limited (ML-RLL) storage is a novel method to increase the recording density and data transfer rate without making changes on optical pickup head. The storage mechanism and experimental system of ML-RLL photo-chromic storage are presented. The mathematical model of ML-RLL storage using photo-chromic materials is proposed. The nonlinear relation between the reflectivity of marks and the exposure power and time is reflected in this model. Based on the model, a proper write strategy used for 4-level run-length limited (4L-RLL) photo-chromic storage is obtained. Then a 4L-RLL photo-chromic dynamic experiment is carried out on 650nm photo-chromic material. Experimental results show that the 650nm photo-chromic materials used in the experiment can be used for ML-RLL storage and the recording channel can be effectively linearized by using proper write strategy, so the conventional signal processing techniques used for linear system can be used.