A ground experimental system was built to study the propellant-tank self-pressurization process of the sub-Newton-thrust N2O monopropellant propulsion system. Numerical simulation using a three-region lumped model was conducted to simulate the same process as well. Simulation results under adiabatic wall condition agree well with the experimental data, which preliminary testifies the validity of the three-region lumped model. Both experimental and simulation results show that the heat compensation for propellant-tank is necessary during the propellant self-pressurization supply process of the sub-Newton-thrust N2O monopropellant propulsion system, in order to keep stability of the tank pressure. It is also found that the initial filling factor, the volume of the tank and the propellant flowrate have great effects on the decline velocity of the propellant-tank pressure.