N2 and CO2 assisted steam huff-n-puff method has been proposed as the primary strategy to develop a new heavy oil reservoir located in the northeast of China having average oil viscosity of 5000mPa·s, reservoir pressure of 14MPa and temperature of 56°C. Prior to field application, a laboratory study from phase behavior to numerical simulation was conducted in this work. The experimental data showed that the swelling of the heavy oil as a result of CO2 dissolution varied almost linearly with pressure, but appeared independent on N2 pressure. CO2 markedly outperformed N2 in swelling heavy oil reducing viscosity and extracting hydrocarbon. In huff-n-puff simulation, averagely 4% and 6% of additional heavy oil was produced by steam injection compared to CO2 and N2 injection after natural depletion. In the scenario of gas assisted steam process, a more noticeable incremental oil recovery (>10%) was produced, which thus demonstrated its potential in this reservoir. The mechanisms of oil recovery stimulation were further elucidated using numerical simulation from the point of parameter variations for pressure, viscosity, temperature, and oil saturation.