Variable Frequency Transformer is a series of flexible AC transmission system equipment, but also a new type of smart grid equipment. Over the past few decades,wind power in China is the fastest-growing emerging renewable energy,however, the problem of frequency stability of power system in the grid-connected wind farms is also caused wide attention.This paper introduces the basic principle of VFT adjust the frequency and the design of rotor speed control,and the simulation model of VFT and wind farm is built in PSCAD/EMDTC,the simulation experiment of the grid-connected wind farms is carried out,the effect of VFT on stabilizing frequency and adjusting frequency is emphatically analyzed.The results show that VFT can improve the transient stability level of the system and increase the power generation capacity.