Based on the corresponding relation of the fourth coefficient q4 from the Zernike polynomial and the focus item from the Seidel aberration, the value of the fourth coefficient q4 from the Zernike polynomial is used as the criterion of the focusing method for laser interferometer. The focusing method using the criterion is feasible and the result of focusing method is steady. Besides, the positive or negative of the q4 can be used to judge whether the piont of intersection of the spherical wave from the interferometer is in front of the focus plane or behind the focus plane of a large aperture off-axis parabolic collimator, which will guide the move direction. The focus plane of the large aperture off-axis parabolic collimator is confirmed by using this method. From the calibration experiment, It′s known that the focusing effect is perfect and the parallelism error of emissive beam from collimator is less than 0.22″.