The invention discloses a microwave aftertreatment modifying method for polymer precursor-derived ceramics. The method comprises the following steps: (1) mixing a polysilazane precursor with a thermosetting agent under nitrogen protection; (2) carrying out thermosetting on the mixture at the temperature of 110 DEG C to 200 DEG C under vacuum conditions; (3) carrying out pyrolysis for 4 hours in a tube type furnace at the temperature of 1,000 DEG C under nitrogen protection; and (4) heating the pyrolysis product to the temperature of 1,000 DEG C by using microwaves under nitrogen protection, carrying out heat preservation for 30 minutes, and carrying out cooling, thereby obtaining modified polymer precursor-derived ceramics. In the prior art, a polymer precursor is modified through conventional heating, a relatively high temperature is usually required, and different degrees of crystallization of a matrix can be caused. The high-temperature thermal stability and electric properties of ceramic materials are greatly dependent on a ceramic amorphous matrix, and the performance of products is always affected by the crystallization of different degrees. Thus, the microwaves are used to carry out rapid low-temperature modification on PDC, and an amorphous state for the matrix is maintained while the performance of the material is greatly improved.