Abstract: The paper introduced basic principle of double-variable control,and described application of VVVF in full-range speed-regulation system with double-variable 6-pulse AC-AC converter.It also analyzed the limitation of traditional VVVF and put forward a method using symmetrical cosine method to reform the waveform of double-varible 6-pulse AC-AC converter.The simulation result showed that the effectiveness of the new control method of VVVF based on double-variable 6-pulse AC-AC converter.