Abstract: Based on the theorem of mass and momentum conservation, this study established an analytical model for variable mass flow, and thusly developed a mathematical expression for the longitudinal pressure distribution along the mainstream of a drip irrigation pipe combined with experimental data. The established model shows that the longitudinal pressure distribution in the drip irrigation pipe is dependent on friction head loss and momentum exchange, and that friction head loss tends to decrease the pressure and the momentum exchange tends to increase it. The solution of the longitudinal pressure distribution model is attributed to determining friction and momentum exchange coefficients. The test results show that distribution index of longitudinal velocity is independent of the characteristic parameters of the drip irrigation pipe, but linear with the number of drip emitters. The expression for the momentum exchange coefficient was obtained by theoretical derivation and regression analysis, and the friction coefficient can be calculated by Blasius formula. The analytical model for variable mass flow and the longitudinal pressure expression for the drip irrigation pipe was then solved. The calculated values of the longitudinal pressure along the drip irrigation pipe agreed well with measured values. In the longitudinal pressure expression, pressure is affected by a pipe-structure parameter (length-diameter ratio)and a flow parameter (entrance Reynolds number). The expression helps guide the structural design and optimize operating conditions, because the two parameters are both directly controllable. This study offers a new thought for the analysis of variable mass flow, and the results would provide a scientific basis for the hydraulic calculation of drip irrigation.