Abstract: In order to simulate the law of nitrogen loss under different weir depths, drainage spaces and drainage depths in underground pipes, field experiment was conducted and nitrogen loss was simulated by DRAINMOD 6.0. The ranges for three factors were set to be 30-80 cm, 8-40m, and 80-120 cm respectively, and the values of three experimental parameters in each simulation were determined with the method of General Rotation Design. The results indicated that the amount of nitrogen loss in drainage tended to be larger as the weir depth or the drainage depth increased, or the drainage space decreased alone. The effects of drainage space and drainage depth on nitrogen loss were greater than those of weir depth. The weir depth was less than 40 cm, drainage depth had greater influence on nitrogen loss than drainage space, and there was an opposite result when the weir depth was deeper than 70 cm. If the drainage depth was the same, the effective way to reduce the nitrogen loss in drainage was to expand the drainage space and decrease the weir depth at the same time.