Due to the effect of gas' non-Darcy flow characteristic and water production conventional binomial productivity equation would not fit for the productivity prediction of overpressued gas reservoir. Based on the conventional horizontal well' s productivity analysis theory, the author defined the gas-water two phase pseudopressure by considering the effect of reservoir stress sensitivity, high-speed non-Darcy and impulsive effect, and deduced a trinomial productivity equation of horizontal well in overpressure gas reservoirs under gas-water two phase upon the method of conformal transforms to. Case study shows the result calculated by new model has a small relative error with that calculated by well test, which indicates a high accuracy. Sensitivity analysis shows high speed non-Darcy flow and impulsive effect have small effect on productivity, while stress sensitivity and water-gas volume ratio will have obvious effect on productivity, meanwhile as the increasing of stress sensitivity index and water-gas volume ratio, the absolute open flow will decrease.