As an alternative for steam stimulation in heavy oil reservoirs，in-situ combustion often faces many problems in thick heavy oil reservoirs，such as low vertical sweep efficiency and channeling combustion. In view of the characteristics and development history of this type of reservoirs，the existing development well pattern based on steam stimulation was used to sidetrack the horizontal section of the vertical oil production wells，and the plane fire flooding was changed to gravity fire flooding. Considering the geological and engineering conditions in the reservoirs，the influence of different factors on the development effect of gravity fire flooding with vertical well sidetracking was analyzed. In addition，the internal stimulation mechanism of this method was studied，and the evaluation model between the recovery factor and the main controlling factors was built with the multiple linear regression equation. The results show that the gravity fire flooding with vertical well sidetracking improves the recovery factor of thick heavy oil reservoirs by 42%. When the sidetracking length of the vertical well is about 1/2 of the well spacing，efficient production can be obtained. Moreover，the evaluation model of recovery factors agrees with the numerical simulation result by more than 85%，guiding the timing of converting production modes and medium-and long-term planning for the field.