Abstract: In order to predict the influence of the back pressure change at the vent of the jet pump on the working characteristic, based on the working characteristic equation of the jet pump under general condition, derived the mathematics expression of the back pressure sensitivity parameter X at the vent, drew the back pressure sensitivity curve, compared it with Brown’s simplified method, analyzed the influence of friction loss factor and density ratio on the back pressure sensitivity. The results indicated:Brown’s simplified method had a better accuracy when the dimensionless flow rate ratio M was lower; the friction loss factor and the density ratio had a significant influence on the back pressure sensitivity.