The research of slippage effect and threshold pressure gradient for gas reservoir is done under low pressure, which is not in accord with the actual situation of the gas reservoir. For the high waste pressure of gas reservoir, the study of the two effects under high pressure is quite vital for the development of compact gas reservoir. By way of indoor high pressure experiments, it can be used to evaluate their effect on development of gas reservoir. The slippage effect will be more serious with the decrease of pressure and permeability. The threshold pressure gradient will be greater when reservoir pressure decreases. But it will decrease as the increasing of porosity. In consideration of high-speed non-Darcy, slippage effect and threshold pressure gradient, the new productivity equation is established. The results show that the threshold pressure gradient will be an obstacle for the gas flow, while the slippage effect is good for the gas flow. The effect of threshold pressure gradient is greater. The new productivity equation is significant for the development of gas reservoir with porosity-crack double medium.