Traditional tubular wind turbine towers may result in a great increase in the fabricating, mounting and transporting cost for large wind turbine systems. A new composite tower was proposed and then the structural optimization was carried out. The new structure is composed of a lattice tower at the bottom with four-angle combined cross-section legs and the steel tube at the top. The stability coefficients curve of the four-angle combined cross-section column subjected to axial compression was first obtained by a series of ultimate bearing capacity analyses. Considering the strength, frequency and slenderness ratio as constraint conditions, the shape and section optimization of the lower lattice tower was carried out. The optimal results show that the proposed structural system can resolve the scarcity of traditional tubular steel towers in transportation and has a 34% less steel consumption.