The development of abnormal high pressure and lowpermeability gas reservoirs is not only affected by stress sensitivity but also influenced by start-up pressure gradient. In view of the particular flow characteristics of this kind of gas reservoir, three flow differential equations are derived which take stress sensitivity, start-up pressure gradient and both of them into account. Furthermore, taking an abnormal high pressure and low permeability gas reservoir as an example, we studied the effects of stress sensitivity and start-up pressure gradient on the productivity. The results show that stress sensitivity and start-up pressure gradient have a great impact on the development of abnormal high pressure and lowpermeability gas reservoirs, and the flow differential equations for this kind of reservoir considering stress sensitivity, start-up pressure gradient and both of the two, have some guidance for the study on flowcharacteristics of this kind of reservoir.