Abstract: An integrated sensing and communication system is expected to be a major application in 6G, but challenges remain in achieving efficient resource allocation and reducing interference between systems. Reconfigurable Intelligent Surfaces (RIS) and Rate-splitting Multiple Access (RSMA) are considered key technologies of 6G, offering potential solutions to these challenges. In this research, an RIS-assisted RSMA integrated sensing and communication system is proposed, introducing a framework for simultaneous communication and sensing using the same frequency spectrum. It explores rate-splitting and joint active-passive beamforming designs to enhance system performance and reduce interference while balancing communication and sensing needs. An optimization problem is formulated to maximize the minimum user rate under the constraint of an effective sensing power threshold. A joint optimization algorithm is proposed to solve this non-convex problem, utilizing a weighted minimum mean square error (WMMSE) approach to construct a rate-WMMSE relationship and employing block coordinate descent and successive convex approximation methods for variable decoupling and iterative solving. Simulation results show that the proposed algorithm achieves a significantly higher minimum user rate compared with other benchmark schemes under the same transmission power budget. This leads to a marked improvement in the fairness of user communications.