A model for simulating the response of monthly runoff from the mountainous watersheds to climatic changes is developed. The model is based on the modifications to the HBV runoff model, and therefore represents the characteristics and runofF generation processes of inland river basins in the arid area of northwest China. Talcing the mountainous watershed of an inland river, the Heihe River originating from the Qilian Mountains and running through the Hexi Corridor as an example, the monthly runoff changes under different climate scenarios are simulated. The simulation indicates that, during the years fmm 1994 to 2030, if the annual mean air temperature increases by 0.5°C, and precipitation keeps unchanged, then the runoff of May and October will increase because of the increase of the snow melt runoff, but the runoff of July and August will decrease to some extent because of the increase of evaporation, and as a result, the annual runoff will decrease by 4%. If the precipitation still keeps unchanged, and the air temperature increases by 1.0°C, in addition to the increase of runoff of May and June, the runoff of July and August will decrease in a larger amount, making the annual runoff decrease by 7.11%. If the air temperature keeps unchanged, the increase of annual precipitation by 10% will cause the increase of runoff by 5.27%; while the increase of precipitation by 20% will cause the increase of runoff by 12.35 %. When the air temperature increases by 0.5°C: and the precipitation increases by 10%, the mnoff will increase only by 1.62%.