The flow field of the surface spillway of an overflow dam and the tilling basin with step down floor at the end of the spillway was simulated by 2-D numerical simulation. The shape of the step down floor was modified from three aspects, namely changing the height of the step  down floor, adding a horizontal platform in front of the step down floor and changing the angle of the outlet. By comparing different plans, the rules of velocity variation near the stilling basin were obtained. The study can be regarded as a reference for engineering design and scientific experiments.