In order to study the important relationship between combining efficiency and reflectivity of dichroic mirrors, rough surface reflection model of multilayer coatings was established. Reflective curves of high-steepness dichroic mirrors for spectral beam combining were calculated and analyzed. Simulation results of combining efficiency were obtained by combining the spectral curves of fiber lasers. The relationship between the surface shape of dichroic mirrors with different angles and reflectivity and combining efficiency were analyzed. The stability of the beam combining system based on dichroic mirrors was simulated and discussed. The results show that, output power of more than 10kW can be obtained by using a small angle dichroic mirror, and combining efficiency can reach 97%. This result is helpful for the improvement of the parameters of dichroic mirrors.