Shearing interferometer in suitable for the determination of the position of image due to its simple structure and easy operation. There is a linear relationship between the shearing fringes appearing inside an interferogram or the direction of the fringes and defocusing. In the interferogram the fringes either do not appear or appear and have the determined direction when the defocusing is zero. The author presents a method of calculating the error of location and analyses the relationship between the error of location and the frequency difference of double-frequency grating (or the amount of shear)and F number of the system. The results of the experiment and the examples of use are also given in this paper.