Nickel-based superalloy C276 had become one of the alternative materials in simulation loop of thorium molten salt reactor(TMSR) nuclear power system. The strength and plasticity decay of C276 alloy under high temperature environment for long time will directly threatened the security of simulation loop of TMSR. The microstructure of C276 alloy after long-term aging at 700℃ was studied by TEM and SEM with EDS, and the high temperature mechanical performance of C276 alloy after aging was researched at 700℃ with an universal high-temperature materials testing machine. At the same time, the morphology of tensile fracture was analyzed by SEM. The results show that massive block μ phase and M₆C carbides are precipitated after long--term aging with 700℃ at the grain boundaries and within grains of C276 alloy, and the block precipitated phase grains grow up with aging time, some precipitated grains at boundaries extend forward the~interior of based crystal. The alloy is strengthened by the effect of precipitation phase, so the strength is not reduced after long--term aging at 700℃ high temperature. As aging time increases, the plasticity of C276 alloy reduces firstly and then increases to the peak after aging 720 h, and decreases again. However, the plasticity of C276 alloy still maintain well after longer aging time. The high temperature tensile fracture mode of C276 alloy is ductile fracture after long-term aging.