A test device for compatibility of a gravity-driven two-phase fluid loop, the gravity-driven two-phase fluid loop comprising an evaporator, a steam pipeline, a condensation pipeline, a liquid storage device and a liquid pipeline which are connected in sequence, wherein the condensation pipeline is located above a gravity field of the liquid storage device; the evaporator comprises a screen mesh evaporator, a liquid distributor and a steam convergence device, and the evaporator is located below the gravity field of the liquid storage device and is coupled to an isotopic heat source; a gravity auxiliary height difference is formed between an inner liquid level of the liquid storage device and a bottom part of the evaporator; and a control valve is disposed on the liquid pipeline; wherein the material of the test device is the same as that of the gravity-driven two-phase fluid loop, and is 022Cr 17Ni 12Mo2 stainless steel, and the working medium is ammonia; wherein the test device is a sealed and hollow gravity heat pipe that is integrally formed; the test device is divided into three sections: a liquid storage component, a connection pipeline, and a high-temperature baking component; wherein one end of the connection pipeline is connected to the liquid storage component, and the other end is connected to the high-temperature baking component; the test device is prepared from the same material as that of the gravity-driven two-phase fluid loop, and the test device is filled with the same working medium as that of the gravity-driven two-phase fluid loop, wherein a pipeline volume of the high-temperature baking component is not less than that of an evaporator of the gravity-driven two-phase fluid loop; an inner surface area of the pipeline of the high-temperature baking component is not less than that of the evaporator; a dimension of a screen mesh filled in the high-temperature baking component is the same as that of a screen mesh in the evaporator; a cross section of the connection pipeline is not less than a maximum value of cross sections of a steam pipeline, a condenser pipeline, and a liquid pipeline of the gravity-driven two-phase fluid loop; a length of the connection pipeline is not greater than a sum of lengths of the steam pipeline and the condenser pipeline of the gravity-driven two-phase fluid loop; a sum of a gas space volume of the liquid storage component and a volume of the connection pipeline is not less than a sum of capacities of the steam pipeline, the condenser pipeline, and the liquid pipeline of the gravity-driven two-phase fluid loop and a gas space volume of a liquid storage device. a pipeline volume of the high-temperature baking component is not less than that of an evaporator of the gravity-driven two-phase fluid loop; an inner surface area of the pipeline of the high-temperature baking component is not less than that of the evaporator; a dimension of a screen mesh filled in the high-temperature baking component is the same as that of a screen mesh in the evaporator; a cross section of the connection pipeline is not less than a maximum value of cross sections of a steam pipeline, a condenser pipeline, and a liquid pipeline of the gravity-driven two-phase fluid loop; a length of the connection pipeline is not greater than a sum of lengths of the steam pipeline and the condenser pipeline of the gravity-driven two-phase fluid loop; a sum of a gas space volume of the liquid storage component and a volume of the connection pipeline is not less than a sum of capacities of the steam pipeline, the condenser pipeline, and the liquid pipeline of the gravity-driven two-phase fluid loop and a gas space volume of a liquid storage device.