The invention discloses a gravity-driven two-phase fluid loop life test device. The process that ammonia is decomposed to generate nitrogen and hydrogen during the life span of a gravity-driven two-phase fluid loop can be effectively simulated so that reliability of the gravity-driven two-phase fluid loop can be guaranteed in a lunar exploration process; meanwhile, calculation of the temperature difference between an evaporator and a fluid accumulator can be tested in the moonlit night operation period of the gravity-driven two-phase fluid loop, and whether the two-phase fluid loop meets the requirement for thermal control of a probe during the life span is judged. The gravity-driven two-phase fluid loop life test device specifically comprises an evaporator heating temperature control loop, a fluid accumulator heating temperature control loop, a steam pipeline heating temperature control loop, a condensation pipeline heating temperature control loop, a condensation pipeline cooling protection module and a temperature data collection and storage module.