A method to measure image noise caused by circuit module was introduced. Firstly

many background star maps were taken at laboratory and calculated statistically

and max gray difference of the same pixel was taken for image noise caused by circuit module. The statistical data show that circuit module noise was accorded with normal probability density distribution. Secondly

based on the energy distribution model of ideal star point

a gray matrix of an ideal star of visual magnitude ~ 6 was calculated. Then star point with circuit image noise was produced with adding random noise matrix of normal distribution to ideal star point. Finally

based on subpixel centroid arithmetic and simulation data

star position error was calculated. The results show that circuit module can bring image noise of normal distribution; For 6 Mv stars at laboratory

this noise can achieve the accuracy level of 1/200 pixel. Compared with the accuracy level 1/25 pixel of actual star position measurement

circuit noise can not be neglected.