Abstract: A method for measuring the size of the dispersed spot of star sensor optical lens is presented. A measurement system consisting of a collimator, a rotary table and a CCD cameral was implemented to measure dispersed spot. After a image is acquired, a method of bibubic interpolation pixel subdivision is used to pick up the gray value of the image, and the size of dispersed spot is calculated by Rayleigh criterion. The actual measurement and verification test show that the measurement accuracy of the dispersed spot in 0.8 FOV reaches 0.5m,repeat measurement accuracy can reach 0.2m.