Abstract: This article deals with three questions;1.To use the SCR units initiated at only zero phase in the part of every period controls temperature and so all escapes the interfence on mains caused by the shifting phase initiating.2.To introduce the constancy prlnciple turns the system of nonlinear variable parameter into the linear atationary system and so removes the interference on the systemic function caused by mains voltage variation.3.It is both simple and accurate to create to get the two or three dimensional state equation and the output equation directly from the time response curve of the controlled object.