Abstract：In order to improve the whole frequency-conversion system, the complex math-model of three-phase frequency-conversion system transforms to the static d-q axis by the PARK matrix.Then it deduces the common expression of the torque characteristics with the equivalent circuit parameters by using the equivalent circuit and bidirectional revolving-field theory of the single-phase induction motor.Finally, a series of tests prove the validity. Key words：frequency-conversion；three-phase induction motor；torque；MATLAB simulink