The study on collimation properties of semiconductor laser beams is valuable and the off-axis model can characterize accurately the output beams of semiconductor laser. Based on Rayleigh-Sommerfeld diffraction formula, the analytical expression of the collimation field of semiconductor laser off-axis beams is derived by the method of stationary phase. According to the expression, it is found that the spot size of collimation beams in two directions is not correlative with transmission distance but proportional to the focal length of lens. On-axis intensity is inversely proportional to the square of the focal length and related to the parameters of optical waveguides. The phase distribution of collimation field is nearly spherical for a short transmission distance, while for a long transimission distance the phase distribution is close to a plane. These results are important in engineering applications.