Abstract: The slippage effect in nano-level pores in shale gas reservoirs makes the seepage mechanism even more complex, and that an analytical solution model was built to quantitatively analyze its influence is of practical significance and theoretical values. Based on trilinear seepage theory for fractured horizontal wells in shale gas reservoirs and analyzing the law of slippage effect on permeability and its calculating relations, a mathematical model was built taking into account the slipped seepage, and a solution was made on the model to obtain an equation of fractured horizontal well productivity, which can be used to forecast the well production. The pore threshold affected by slippage effect is defined based on the increase range of permeability and the value of production increase. The model was used, through example calculations, to analyze the slippage effect on the increase of productivity under different pore diameters, different production pressure differential, and quantitatively evaluate the magnitude of slippage effect. The result shows that the initial productivity may reach 1 500 m3/d, and the production at later stage may also reach 400 m3/d. Therefore, the slippage effect should be taken into consideration in productivity forecast when the pores of shale gas reservoir are small, as to reflect its seepage regularity more scientifically and comprehensively.