Based on the theory of nonequilibrium statistics and density operator equation, the generalized master equation satisfied by characteristic function for the nonextensive reaction-diffusion systems affected by pressure is derived by calculating the time variation of probability distribution function, where the pressure of the nonextensive systems is given in the framework of Tsallis statistics. This new equation not only depends on the nonextensive parameter but also has more nonlinear terms as compared with the master equation in the phenomenological theory, and is thus more general.