The invention provides a subtree-based multicasting traffic grooming method in a WDM (Wavelength Division Multiplex) optical network, belonging to the technical field of network communication. The method comprises the steps of: initializing, searching an optimal subtree, grooming by using the optimal subtree, adding a subtree, newly building a subtree, allocating resources, and releasing corresponding resources. Under the condition of multiple restrains, the method is used for carrying out service grooming by using the existing optical tree and the optical path in the network, has wider applicable range, low requirement on functions of network nodes in the WDM optical network, and has strong dynamic configuration traffic grooming capability due to no need of the network nodes compared with the virtual topology based traffic grooming method, and a relatively simple structure of the needed node.