This paper investigates the influence on frequency of delivering large-scale wind energy and thermal power by AC-DC parallel transmission systems. A two-area system with four generators, large-scale wind farms and HVDC line between area 1 and area 2 was built up in PSCAD/EMTDC. Simulations were carried out under the circumstances of using wind energy replace thermal power, the results showed that the capability of frequency control of the thermal power units are better than that of wind generators. Finally, automatic generation control(AGC) was added to generator 2 in area 1, the AGC was designed taking into account both the AC line power changes and the DC line power changes, and the frequency characteristics of the system and it’s stability were improved.