This article introduces a new method for dynamic spectral study of a semiconductor under its optical nonlinearity condition. With this method, a semiconductor GdS in the optical nonlinearity is investigated, it can be observed that the CdS's transmitting spectra (at the edge of the absorbing band) are varying with time (at the ms degree). The computer imitating calculation consists with the experimental result. The conditions have been found for realizing the room temperature optical bistabi-lity on CdS.