The functions of forest regulating fluvial runoff on north slopes of the Tianshan Mountains were analyzed by using 31 basins of hydrological, weather and forest data and so on. The results were as follows: forest and evaporation could reduce the functions of annual runoff discharges and the regulating function of fluvial runoff by forest was far more than that of evaporation. Among the forest factors, the effects of increasing forest coverage was larger than the unit stock volume increased on fluvial annual runoff. Under the natural environment on north slopes of the Tianshan Mountains.When the forest corerage was increased by 1?%,the fluvial annual runoff was decreased 0.33?mm on average. When the unit stock volume was increased by 1?%.the fluvial annual runoff was decreased 0.27?mm on average.