PSTM is an optical microscopy which can produce superior resolution. The lateral resolution is determined by the diameter of the probe tip. But the tip figure also contributes to the resolution. In experiment, it has been found that the coupling efficiency of the parabolic tip is much higher than that of the truncated tip. In order to optimize the tip fabrication ,we systematically analyzed several kinds of tip figures by considering their energy coupling and model coupling. The numerical result is given.