It is difficult to describe actual seepage characteristics of low permeability gas reservoir by using conventional binomial productivity equation due to effects of stress sensitivity and start-up pressure gradient. Starting from seepage flow mechanics theory and based on the principle of conformal transformation and equivalent percolating resistance, established is trinomial productivity equation for horizontal gas well with consideration of stress sensitivity, start-up pressure gradient and high velocity non-Darcy effect, conducted is new definition of pseudo pressure considering stress sensibility, developed is a method for calculating pseudo pressure under low, intermediate and high pressure conditions and derived is a calculation formula for absolute open flow rate. Analyzed is effect of correction coefficient C for stress sensibility and start-up pressure gradient and different stress sensibility coefficients on absolute open flow rate. Productivity of horizontal gas well cannot be analyzed by using conventional binomial productivity equation when it has negative slope and the trinomial productivity equation can solve the problem.