In large scale optical networks, great challenge are brought by the fast growing of multicast services. Multicast lightpath-tree may fail to work in data plane due to the low OSNR (optical signal to noise ratio) value, even though the signaling procedure is successful in the control plane. That is called “false signaling-success” in control plane. To solve this problem, an OSNR Constrained Multicast Lightpath Provisioning Mechanism (OC-MLPM) was proposed. The OC-MLPM makes advantage of the OSNR monitoring function in each node, and is able to allocate the wavelength by judging the OSNR value carried in signaling message in order to make sure lightpath could be established smoothly in both data plane and control plane for multicast service. Simulation results show that the OC-MLPM is able to solve the “false signaling-success” problem, and to reduce the blocking probability effectively as well.