Laser diode is the preferred laser source for satellite laser communication system, and it is significant to analyze its reliability in space radiation environment. According to the character of space radiation and devices′ radiation dose, normal distribution is used to describe both space radiation dose rate and performance degradation per unit radiation dose, and the reliability function of laser diode is achieved. The performance degradation in high- orbit radiation environment is simulated. On the basis of the simulated data, devices′ distribution function is evaluated by maximum likelihood estimate, and then the reliability curve and average life are obtained. The feasibility of using less experimental time and less sample number to evaluate distribution function is analyzed.