Abstract: For power conversion unit of the high temperature gas-cooled reactor (HTR), three combined cycle plans were designed. They were compared with closed Brayton cycle at the reactor outlet temperature of 900 ℃. When the reactor outlet temperature is higher, the closed Brayton cycle has a higher reactor inlet temperature which is limited by material of the reactor vessel, and cycle needs a larger compression ratio. In combined cycle, since the reactor inlet temperature is lower than 370 ℃, SA533 steel can be used as reactor vessel without cooling the inner wall. The combined cycle needs a smaller compression ratio. The plan comparison shows that improving the cycle efficiency of the combined cycle needs to increase the bottoming cycle output power. In plan 3, the topping cycle is a simple Brayton cycle, and the bottoming cycle is a reheated Rankine cycle. The efficiency of the combined cycle 3 can obtain 50.1%.