Based on the traditional geometric optics principles and the method of ray tracking, this paper reports the calculation of the aberration of the mode index lens (MIL) when the collimated beam incidents upon it inclinely with an angle, and the divergency of the output beam from the MIL when it works as a collimator for the end-butt coupling between the semiconductor laser and the waveguide. The results demonstrate that as a collimator and converging lens in the integrated-optic spectrum analyzer (IOSA) the MIL is suitable.