For deeply understanding of TOPMODEL hydrological model and influence of TOPMODEL parameter choosing on flood simulation, the global sensitivity analysis methodology, Sobol′ method, was used to give a quantitive study on the effects of TOPMODEL individual parameter and parameters combination on flood predicting. The results show that all the parameters of TOPMODEL have poor individual influence on Nash efficiency coefficient, relative error of runoff amount and relative error of peak flow; but the effective lateral saturated transmissivity, hillslope routing velocity and form of the exponential decline in conductivity have big individual influence on peak flow time. The two parameters combination influence of exponential decline in conductivity and unsaturated zone time delay has big influence on Nash efficiency coefficient, relative error of runoff amount and relative error of peak flow; the combination effect of exponential decline in conductivity and effective lateral saturated transmissivity has important influence on peak flow time.