This paper describes the development of a simulation model in state space representation for a class of microwave heating model. Under the assumptions on thermodynamics mechanism, one-dimensional heating model in rectangle waveguide is derived by combining Lambert's law and heat conduction equation. Through introducing the spatial differential operator in inhomogeneous thermal conduction equation, one-dimensional heating model in form of partial differential equations (PDE) can be divided into finite-dimensional slow one and infinite-dimensional complement, which provides a application for Galerkin's method to transform the PDE into state space form. The simulation of state space model has shown that the trend and distribution of temperature are in good agreement with that of traditional numerical model.