Abstract: Aiming at the characteristics of low bubble point pressure, high structure amplitude, having a heavy oil cushion at the bottom of the reservoir and developed interlayers in Donghe-1 Carboniferous reservoir, equivalent numerical models are established to simulate the gas injection reservoir development mechanism after waterflooding, to analyze the errors of component models converted at different time, to determine the reasonable conversion timing and improve the efficiency and accuracy of the models. By comparing the differences between conventional grid and hybrid grid methods to deal with the heavy oil cushion, the paper describes the shape of the heavy oil cushion, analyzes the influences of the cushion on production performance and improves the history matching accuracy. The equivalent models are used to rapid simulate the gas injection process after waterflooding and to compare the differences between different injection media and different injection modes, providing basis for the establishment of actual reservoir model and study of gas injection mechanism. The study results show that the equivalent models can significantly improve the research efficiency; the hybrid grid method is more accurate in describing the shape and vertical properties of the heavy oil cushion; at the end of waterflooding, about 90% of the fitting time can be saved and the error can be controlled with the relative errors of main indicators less than 1% by changing the oil-water two-phase or three-phase black oil model into the component model. The mechanism study indicates that during reservoir development converted from waterflooding to gas injection, interlayers could inhibit gas channeling to some extent. Influenced by gravity and with the vertical dynamic balancing of gas-oil-water three phases, a new oil ring may occur at the bottom of secondary gas cap. When gas-oil ratio exceeds 200 m3/m3, gas yield should be controlled to increase gas injection efficiency