Introduces the double-variable control principle and a method to realize stepless speed control with the combintion of variable-frequency and voltage regulation. Sets up the simulation model of double-variable Ac-Ac frequency converter based on MATLAB. Presents a new control method (improved square-wave law) that achieves closed-loop control easily and ensures the output voltage with a good symmetry and sine. The conclusion indicates that the method using the improved square-wave law for variable-frequency and voltage regulation reduces the output waveform's harmonic components and improves frequency converter's performance.