This paper focuses on interference suppression in Reconfigurable Intelligent Surface (RIS)-assisted multicast communication, where a multi-antennas base station (BS) transmits identical messages to a group of single-antenna users, while a multi-antennas jammer simultaneously sends interference signals to the users. A RIS consisting of many reflectors is deployed near BS to enhance the multicast channel and suppress the interference channel by carefully designing reflection coefficients. Under such settings, a signal to interference plus noise ratio maximization problem is formulated to maximize the achievable rate by designing feasible reflection coefficients at the RIS and a beamforming vector at the BS. Then, an alternating optimization combined with semidefinite relaxation and bisection search over a series of the solutions of feasibility problems is adopted to iteratively optimize the reflection coefficients and beamforming vector separately. At last, simulation results show the RIS can significantly improve the multicast rate with interference.