Provided are a device and a method for testing the compatibility of a gravity-driven two-phase fluid loop. The test device and method can effectively simulate the decomposition process of an ammonia working substance in the gravity-driven two-phase fluid loop in the case of a high temperature with nickel as a catalyst to judge the compatibility of the gravity-driven two-phase fluid loop. Firstly, a test device for simulating the decomposition process of the ammonia working substance in the gravity-driven two-phase fluid loop during a moon day is designed, then the amount of non-condensable gases produced during a moon day from the ammonia working substance in the test device is measured using a method for testing a non-condensable gas in a gravity assisted heat pipe, and then a high-purity nitrogen in the same substance amount as that of the non-condensable gases is filled into the gravity-driven two-phase fluid loop to achieve the determination of the compatibility of the gravity-driven two-phase fluid loop. The test device is simple, and the test method is rapid, convenient and effective.