Variable frequency transformer(VFT) is a series of flexible AC transmission system equipment, and it also is 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. The basic principle of VFT adjust the frequency and the design of rotor speed control was introduced, and the simulation model of VFT and wind farm was built in PSCAD/EMDTC, the simulation experiment of the grid-connected wind farms was carried out, the effect of VFT on stabilizing frequency and adjusting frequency was emphatically analyzed. The results show that VFT improve the transient stability level of the system and increase the power generation capacity.