To study the development of gravity drainage, physical simulation experiments were conducted with a high-temperature and high-pressure 3D physical simulation system for a mid-deep extra-heavy oil reservoir. The characteristics of steam chamber development and production dynamics in SAGD processes under different pressures were studied to optimize the reasonable production pressure for SAGD development in a mid-deep extra-heavy oil reservoir. The development effects of SAGD and MFAGD (Multi-thermal Fluid Assisted Gravity Drainage) were compared, and the influence of gas-water ratio on the development effects of MFAGD was studied. On this basis, a gravity drainage method, SAGD-MFAGD, is proposed to efficiently develop high-pressure heavy oil reservoirs, and the gas-water ratio in the MFAGD stage is optimized. The research results show that the production pressure of SAGD has a significant effect on steam chamber expansion and recovery performance. At higher production pressure, the steam chamber expands slowly, and at the initial stage of production, the steam chamber is elliptical and the oil production rises slowly. After the steam chamber rises to the top of the reservoir, the upper part of the steam chamber expands to both sides at a faster speed, and the steam chamber is shaped like a funnel. The higher the production pressure, the higher the temperature of the steam chamber, the lower the residual oil saturation in the steam chamber, and the higher the recovery rate of SAGD production. However, the temperature of produced fluid increases with the increase of production pressure.If the temperature of the produced fluid is too high, it will damage the production equipment. Based on the oilfield conditions,it is recommended that the production pressure in SAGD process is 5~7 MPa. In the early stage of MFAGD, oil production increases rapidly, while in the later stage of MFAGD, oil production decreases slowly, and instantaneous oil production and the instantaneous oil-steam ratio are higher. With an increase of the gas-water ratio, the oil recovery and cumulative oil-steam ratio in MFAGD both increased. However, when the gas-water ratio exceeds 50, the increase of oil production and cumulative oil-steam ratio slows down. So the recommended gas-water ratio for MFAGD is 50 at a production pressure of 5 MPa. Under the same production pressure, conversion of SAGD to MFAGD in the later stage can improve recovery by about 5%. In the initial stage of SAGD to MFAGD, a higher gas-water ratio is recommended, which will gradually decrease in the later stage.