Relief well is one of the main reinforcement measures of a levee. Effects to relieve pressure depend on mechanical clogging and chemical clogging in a relief well. In North River Levee, there are 383 existing relief Wells, located in Shijiao, Datang, Huangtang, Xinan levee reaches etc., of which 147 relief wells in Shijiao Levee were constructed during 1966-2001. The techniques of drilling holes contain slurrysupported drilling method and steelcasing vibrating sinking method, while the filter hole can be grooved or wrapped by yarn. For the different outflows of wells, the effect to relieve pressure varies during flood season. According to the measured data of piezometric heads and discharges of relief wells during flood season, pumping tests, field test results and numerical simulations, the conclusion that the mechanical clogging is a main measure can be obtained. As to relief wells in North River Levee, the cases of 70% outflows of wells show perfect effect to relieve pressure at Lianoutang Segment and, only the cases of 20% show weak effect and safety hidden troubles at Hengji Segment. So according to the characteristics of strata, we should choose appropriate welldrilling techniques to ensure the effect of relief wells and the safety of levees.