To fulfil in-vessel maintenance in the China Fusion Engineering Test Reactor (CFETR), a Master-Slave remote handling system with high reliability, stability, and flexibility is required. Due to the complexity of CFETR's internal environment and the robot system, operators need to be well trained before any physical maintenance is conducted. It is of high risk, high cost, and low efficiency that operators are trained in physical environment. The above problems can be avoided when operators are trained in a virtual environment. However, it is difficult to achieve the realization in the master side and contact force in the slave side. In this paper, we develop a new Virtual Assembly Simulation Platform with high quality realization of the contact force and the operator's telepresence. The platform consists of a master-slave robot system and a simulation environment build upon CoppeliaSim. To remove the influence of the tool's gravity on the feedback force obtained by the force sensor installed at the end of robot. We propose a least square method-based gravity compensation algorithm. The replaceability of the virtual platform to the physical platform and the effectiveness of the algorithm are demonstrated by the peg-in-hole assembly experiment.