The structural design, heat transfer capability analysis, ground equivalent validation, and on-orbit flight of the two-phase fluid loop based on flat-plate evaporation module of the Chang'e-4 detector are introduced. Within the temperature range of - 30 degrees C similar to -10 degrees C, the designed two-phase fluid loop has a heat transfer capacity of greater than 200 W. An equivalent test prototype is designed and manufactured to examine the heat transfer performance of the two-phase fluid loop under the ground condition of 1 g. The driving force of the equivalent test prototype is less than that of the flight prototype, while the flow resistance is equivalent to that of the flight prototype. The test heat transfer capacity is smaller than that of the flight prototype on the lunar surface. According to the equivalent test prototype, the heat transfer capacity is no less than 130 W, which meets the requirements of Chang'e-4. During the 14-moon day- night cycle, the temperature of the two-phase fluid loop gradually decreased to an equilibrium value of -10 degrees C. During the wake-up process of the detector in moon day, the control valve was closed, and the temperature of the flat-plate evaporation module rose rapidly, indicating that the function of blocking heat transfer is normal.