Present threshold pressure gradient test cannot characterize the present situation of real threshold pressure gradient. After back pressure control system with high precision was introduced，experimental approaches were carried out to evaluate the threshold pressure gradient in the tight gas reservoir under reservoir conditions. Compared with the measured result under normal pressure，the threshold pressure gradient obtained by the new method is lower under the condition of same permeability and the difference becomes larger when core permeability is lower. Results show that the threshold pressure gradient is not a constant during the development of reservoir，but varies with the pore fluid pressure，and it is called dynamic threshold pressure gradient. New concepts of threshold pressure gradient sensitivity and threshold pressure gradient sensitive coefficient were proposed and defined to describe the dynamic threshold pressure gradient in the tight gas reservoir. The influencing factors of dynamic threshold pressure gradient were experimentally studied as well. Results indicate that the threshold pressure gradient of the tight cores show a linear increase with the decrease of pore fluid pressure. When the core permeability is lower and the water saturation is larger，the threshold pressure gradient becomes larger under the condition of reservoir and the variation range of the threshold pressure gradient during the development is larger. The threshold pressure gradient sensitivity is stronger and the dynamic variation of threshold pressure gradient is more obvious.