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A mechanically pumped two-phase loop (MPTL) system used for the accurate and stable thermal control of orbital heat sources can show excellent characteristics. In order to study the dynamic behaviors of heat and mass transfer of MPTL systems, particularly in response to heat load variations, a transient numerical model was developed by using the time-dependent Navier-Stokes equations. A comparison between the simulation and test results indicated that the errors of mass flow rate were at around ±10%, of which the validity and accuracy were verified. The model was used to study the operating state, and flow and heat characteristics on the basis of the analyses of variations in mass flow rate, temperature, and quality under different operating conditions. Above all, the complex transient behaviors in response to heat load variations in an MPTL system were studied in this model, such as the mass transfer between the accumulator and loop. Results indicate that the phenomenon of mass exchange occurs between the main loop and the accumulator when the heating power increased or decreased. Variations in temperature and pressure in the accumulator were different for the cases of increasing and decreasing power. The slope of the exchange rate curve and the maximal value of the flow rate decreased with the increase in filling amount. The model could be used to guide the design of MPTL systems and to predict the behavior before a system is built.