Optical network-on-chip (ONoC) is an emerging architecture to realize high-performance and low power consumption for many-core processors. Multicast in ONoC is a vital communication pattern widely used in parallel computing and computational genomics. This paper proposes an efficient scheme for multiple multicast requests in ONoC. Firstly, we design an algorithm to construct two trees for two multicasts without conflicting edges in a mesh topology, and it only uses one wavelength. We propose a graph to represent the relationship between each multicast. Secondly, a group-based wavelength assignment scheme is designed to assign wavelengths for multiple multicasts. Finally, we used data analysis to extract helpful simulation results to form conclusions. The results indicate that our algorithm is superior to other tree-based schemes in communication latency and wavelength number usage.