The sediment will deposit on the surface of irrigated soils when the muddy water is used to irrigate. The deposition layer will change the surface features of irrigated soils. Therefore infiltration features of muddy water is different from that of clean water. But these problems have been only researched by the way of experiment and there are not any models with physical bases to forecast the infiltration processes of muddy water. In the paper the original Green-Ampt Equation was modified based on the analyses of infiltration features of muddy water. Effect of deposited sediment on infiltration was reflected by the average suction of wetting front. And the modified Green-Ampt Equation was convenient to calculate the infiltration process through existed infiltration data for clean water infiltration. The modified Green-Ampt Equation was checked by lab experimental data and the result indicated that the model is reasonable.