A mathematical model of effective radius for low velocity non-Darcy flow in low permeability composite reservoir with two different zones is established and the threshold pressure gradients of the two zones in low permeability composite reservoir are taken into consideration. The solution of bottomhole pressure is obtained, the type curves of pressure behavior for low velocity non-Darcy flow in composite reservoir are plotted, and the effects of parameter on the pressure behavior, such as threshold pressure gradients, diffusivity ratio, mobility ratio, and interfacial radius, are discussed. It is shown in the study that the type curve of pressure derivative is no longer horizontal line at the period of radial flow in the inner and outer zones, but it tends to be upward gradually for the effect of threshold pressure gradient. This characteristic is different from the traditional composite reservoir with two zones. The model is used to interpret the actual well test data of low permeability reservoir, and interpretation results are in good accordance with the actual pressure behavior, indicating that it is practical for studying the characteristics of low permeability reservoir.