To measure the roll angle between two spacecrafts at the last-phase of spacecraft-docking accurately, a roll angle measurement system based on square wave magneto-optic modulation was present. A model of the modulated signal was present according to the Faraday effect and Malous law, and the relationship equation between the modulated signal and the roll angle was established according to the character of modulated signal. The combination of the variation trend of the modulated signal was used to delete the extraneous roots of the equation, then the roll angle measurement model based on square wave magneto-optic modulation was acquired. Simulation results indicate that the new method has higher measurement precision, and the measurement scale is -90~90°, wider than that of the traditional method. Furthermore, the new method shows advantage on data acquisition and data process.In conclusion,the method can provide a new reference for measuring the roll angle between spacecrafts in higher precision and wider scale for spacecraft docking.