Abstract: The existence of threshold pressure gradient in the flow process of low permeability gas reservoirs makes its flow mechanism more complicated than that of the conventional gas reservoirs. Moreover, due to acidizing, fracturing and some inherent features of gas reservoirs during the development of low-permeability gas reservoirs, there are some differences in the rock and fluid properties between the near wellbore areas and far-from-wellbore areas, resulting in the formation of composite gas reservoirs. Therefore, the paper establishes a unsteady flow model considering the threshold pressure gradient for horizontal wells in composite reservoirs, and obtains dimensionless analytical solutions of the model by using complex mathematical methods such as Laplace transformation and orthogonal transformation. Based on which, the influences of threshold pressure gradient, horizontal section length, mobility ratio and inner radius on the dynamic characteristics of the unsteady pressure in horizontal wells in the composite reservoirs are also studied. The results show that the pressure response type curves of the proposed model can be divided into 7 stages such as wellbore storage stage, transitional flow stage after wellbore storage, early vertical radial flow stage, mid-term linear flow stage, inner-zone pseudo-radial flow stage, transitional flow stage from inner zone to outer zone and outer-zone pseudo-radial flow stage. The higher the threshold pressure gradient, the larger the up-warping amplitude of the pressure response type curve; the longer the horizontal section length, the lower the position of the type curve; the bigger the mobility ratio, the higher the position of the type curve; the larger the inner zone radius, the longer the duration of inner-zone pseudo-radial flow stage will be. This research enriches the theories related to unsteady flow in low-permeability composite gas reservoirs, and provides a theoretical basis for a better understanding of the flow characteristics and for efficient development of gas reservoirs