Taking energy and entropy as the main parameters, the changing process and dissipative properties of a laser system both with and without outside influence are considered. With the help of the principle of minimun entropy production, we introduce the phenomenolo-gical linear law to explain quantitatively the relations among the stimulated radiation, the stimulated absorption and the photon number density inside the cavity and to seek the thermodynamic criterion of the steady state of a laser under entropy and energy flux exchange with the environment.