It is a very important technical question whether water injection string can rely on string weight to go through the well hole or not. Considering the gap and random contact between the string and the well wall, the mechanics analysis model of water injection string is established and nonlinear mechanics analysis is done by using " gap element of multi-direction contact and friction " theory. The analyzed result of three wells in Daqing oil field shows that the maximum cumulative friction of is 0.74kN. It means that water injection string can put under the bottom of well relying on string weight.