The two-phase mechanical pumped cooling loop is a promising thermal control system which is just like a heat pipe to realize heat extraction from ener-electrical equipment by phase change efficiently. In order to apply this technology for the space thermal control systems, transient tests have been performed to clearly comprehend its performance and to characterize operation in different conditions. Special attention has been paid to the system dynamic working characteristics under different conditions such as variation heat load and mass flow. According to the experimental findings, the transient liquid transfer between the main loop and accumulator was analyzed to explain the transient phenomena during the experiments. From the experiments, it is demonstrated that when mass flow increases or heat load decreases, work fluid would flow out of the accumulator because of vapour quality changing. On the contrary, when mass flow decreases or heat load increases, work fluid would flow back to the accumulator .The paper also presents that accumulator is just like the “brain” of the system and each action of the vapour-liquid interface would cause the temperature or pressure change of the evaporator.