Existing models of threshold pressure gradient for formation water induce non-negligible variations in calculation results. In order to solve this problem, cores from an ultra-deep carbonate reservoir in the Puguang gas field at the Feixianguan formation in the Sichuan basin are selected to test the formation water threshold pressure gradient under reservoir conditions. A model of threshold pressure gradient for formation water is established based on experimental results. Results from different models of threshold pressure gradient are compared. Main factors affecting threshold pressure gradient for formation water are analyzed. A case from the Puguang gas field is investigated. Results demonstrate that the threshold pressure gradient for formation water increases with the decrease of permeability. The threshold pressure gradient rises rapidly if permeability is extremely low. Temperature can affect viscosity of formation water. The non-Newtonian characteristics of formation water in small pores can be influenced. As a result, threshold pressure gradient for formation water is affected. Permeability and viscosity of formation water are the main influencing factors for the threshold pressure gradient for formation water. The threshold pressure gradient for formation water in the Puguang gas field is significant. Therefore, it should be considered in the studies of the water intrusion mechanism.