The utility model provides a diameter -variable cast in situ concrete wind turbine generator system pylon step by step contains upper portion steel tower segment, lower part cast in situ concrete tower segment, prestress wire subassembly and reinforced concrete base. Upper portion steel tower segment passes through the prestress wire subassembly and links to each other with lower part cast in situ concrete tower segment, lower part cast in situ concrete tower segment is the flagpole formula tower segment of reducing step by step, and the rigid coupling is on the base, the anchor of prestress wire subassembly upper end is at the bottom of the steel tower segment on the flange, and the lower extreme anchor is in the base. Performances such as steadiness, durability, wholeness and the shock resistance of this pylon are all better, not only overcome aspects such as traditional steel tower frame large diameter manufacturing, transportation and hoist and mount and retrained, can improve the wind resource utilization rate in low wind speed area moreover, promoted the generated energy to can save the form cost with and steel, be the novel wind generator pylon of the convenient and economic environmental protection of a safe and reliable, construction.