Abstract: Among numerous space nuclear power sources, the high temperature gas-cooled reactor with closed Brayton cycle has the advantages of both high power and high energy conversion efficiency. An open-grid megawatt gas-cooled space nuclear reactor (OMEGA) was investigated in this paper. It mainly consists of three parts: a heat source provided by the reactor core, an energy conversion system converting heat into electricity, a heat rejection system as heat sink. Generally there are four structural layouts of gas-cooled space reactor (GCSR), including porous prism core mainly used for nuclear thermal propulsion, pin-block type reactor, pellet bed reactor and open-grid type reactor. Not alike the other three, there is no solid matrix in open-grid type reactor. Consequently, it is hard for the core removing fission and decay power through heat conduction under the transient condition of core coolant flow reduction or even totally loss. Due to the absence of relevant research, passive decay heat removal capability during shutdown transient condition was investigated in this paper. Especially radiation heat transfer among fuel rod claddings was calculated. Furthermore, a system transient analysis code of start-up and shutdown (TASS) was developed by staggered grid technique and solved by GEAR algorithm, including core, turbine-generator-compressor, regenerator, condenser, heat pipe radiator and other modules. TASS was verified by comparing the design value with the program calculation value, and the transient conditions of system start-up and shutdown were simulated by TASS. The results show that the core flow and power match well by inserting positive reactivity in two stages and elevating the rotating speed of turbomachinery in a ladder-type. The start-up process of the system can be accomplished in 3.5 h, and the steady-state operation of reactor power 3 406 kW and flow rate 14.2 kg/s can be achieved. At the beginning of start-up, an additional power supply of 5 kW is required for the turbomachinery. During scheduled shutdown, average temperature of fuel element experiences a trend of decline, rise and fall. During emergency shutdown, pellet temperature continues to drop and cladding temperature is slightly lower than pellet temperature. No matter it’s a scheduled shutdown or emergency shutdown, due to the existence of radiation heat transfer among fuel rod claddings, maximum temperature of cladding and pellet is lower than the safety limits of their materials, which reflects the passive safety of core design. And as a result of the Biot number of fuel element is lower than 0.1, cladding temperature is close to the pellet temperature, which requires special attention.