Based on the field observation and simulation in laboratory,the effects of frozen stagnant water (FSW) on ecoenvironmental construction of Northwest area were analyzed. FSW is seasonal solid underground water, formed by freezing effects in winter in the frozen soil layer of independent water and salt dynamics.It has positive and negative effects on ecoenvironmental construction. FSW generally increases the amount of water by 8%～25%.The maximum water content is 39.51%.The water content is 300% at frozen lumps on the ground. It is indispensable or even the unique water source for natural forest and manual vegetation lived through the winter, vegetation growth in spring, water metabolism, and also is a central factor of ecological balance in Three-North area. When the FSW content ＜5%，it brings about contabescence for natural forest and manual vegelation, and acceleration of desertification but when water content arrives at immoderate sturated state, some geological phenomena as frozen lumps on the ground, landslides, dents and salingation of swamplifotion are formed, thus accelerating the deterioralion and imbalance of ecology. The control of FSW content is very important to building the engineering of sustainable developing forestry and improving ecoenvironment in Northwest area.