The imaging law of diffraction-limited spiral phase contrast microscope under coherent illumination condition is derived. Three parameters of spiral phase plate that influence the imaging quality, i.e., central size, size and step number, are pointed out. Exhaustive investigations into the influences of the three parameters on the imaging results are made, and their reasonable values are presented. The validity is confirmed by experiments. This method is useful in designing the spiral phase plate. Moreover, it is also valid for designing the filters based on spatial filtering.