Taking theories and stimulation as the starting points, this paper gives the optimal design for dichroic plate in the advanced technology of imaging diagnostic. Dichroic plate is firstly analyzed theoretically, whose result shows that it is a wavelength-sensitive device, different from the traditional understanding of dichroic plate. In fact, a dichroic plate is a wavelength selective, and its outward manifestation is a frequency selective plate. There are two ways of putting medium into the dichroic plate. It can be seen that with the increase of the medium's semidiameter, the passband bandwidth increases obviously and the rising slope is sharper. When the medium is fully populated, the passband bandwidth reaches its maximum, the freguency increasing upto 12 GHz, and in the meanwhile the rising slope possessing the top speed. The other is to adopt the method of coating. If a hollow dielectric cylinder is filled to become a waveguide, it can also be seen that with the thickening of the wall of the dielectric cylinder, the passband bandwidth also increases obviously and the rising slope is sharper. When the medium is fully filled in the circular waveguide, the result is the same as that of the first method.