Using the Wigner operator in coherent state representation and the technique of integration within an ordered product of operators (IWOP), the Wigner function of the thermal coherent state was obtained. By adopting the coherent state representation and normal product form of operators, the Wigner function of the corresponding mixed states was obtained too. It is found that the Wigner function of the thermal coherent state agrees with that of the corresponding mixed states, which support the thermofield dynamics laws. And it is convenient to study the Wigner functions of quantum states by the coherent state representation, IWOP technique and normal product form of operators. The results provide further new insights into the phase space technology of quantum statistics and thermo field dynamics, and have certain theoretical guidance meaning for studying other quantum phase-space distribution functions.