Joint transform correlator is a technology used on space camera image motion measurement. The traditional optical joint transform correlator implemented by space light modulator is replaced by electronical joint transform correlator implemented by running the algorithm in a digital processor, which makes correlator adapt to the space radioactive environment and enhances the reliability. By doing massive simulation experiments, the influence of the size of the input images and the relative movement between two input images on the measurement precision of joint transform correlation is analyzed. Results of simulation experiments show that it is necessary to enlarge the size of the input images to increase the precision and sensitivity of joint transform correlator when designing a correlator, and enlarging size of the input images can also prevent singular values showing in the measurement results.