This paper investigates the defects that exist in the blocking probability and the cost of establishing light-tree in the multicast routing algorithm with sparse splitting capability constraint based on the Virtual Source (VS). It proposes a new multicast routing algorithm N_VSBA (New Virtual Source Based Algorithm). By introducing the minimum interference routing algorithm in the path calculation among the VS nodes, this new algorithm solves the problem of blocking caused by establishing the light-tree through a pre-computed path when two VS nodes concentrate. This algorithm also solves the problem of VS-based algorithm’s not being able to utilize the TaC (Tap and Continue) capability of indecomposable optical nodes by introduction of MPH (Minimum Path-cost Heuristic) algorithm into the process of adding light-tree into indecomposable nodes. Simulation results show that the new algorithm has improvements of different degrees in aspects of blocking probability, average number of wavelength links and average maximum number of wavelength.