Abstract: The amount of fluvial runoff and the length of low runoff period lays very importance influence upon the production of industry and agriculture and the social life of the people in the arid desert area in Xinjiang Uygur Autnomous Region. The comprehensive study on the relationship between the forest coverage and unit stock volume and the amount of fluvial runoff of rivers in the dry period indicated that the increment of forest coverage in the mountainous areas could increase the amount of fluvial runoff of large rivers in the dry seasons and shorten the low runoff period. As to the rivers on north slope of the Tianshan Mountains, the low runoff period was shortened by 2 month because of increment of the forest coverage in the catchment areas, the monthly average amount of fluvial runoff during the dry seasons was increased by 3.5×10 8?m 3 and the fluvial runoff in the driest month was increased by 0.7×10 8?m 3.When the ratio of forest coverage was increased by one percent, the fluvial runoff in the driest month was increased by 400×10 4?m 3.When the forest unit stock volume was increased by 1?000?m 3, the fluvial runoff in the driest month was increased by 2?700×10 4?m 3.