A novel method of fourth order Runge-Kutta with variable step is applie in calculating the flood routing and water line in reservoir. In comparison with the traditional trial method through solving hydraulic differential equations,The method can effectively avoid missing the peak.Taking a reinforcement project of a small reservoir in Anhui as an example,the paper calculated flood routing and water line of the spillway by using tradition method and varying step four order Runge-Kutta method and got the flood hydrograph, water level change curve and surface line of spillway.The result showed that the proposed method is easy to program design and meet the precision requirement of engineering,and can be used in the calculation of hydrological differential equation.