The rotational inertia of a rigid body (aero-engine, automotive powertrain, etc.) is closely related to the system’ s dynamic performances in terms of the vibration, structural durability, stability and so on. It is of great significance to improve the measuring accuracy of the rigid body rotational inertia for the system dynamic analysis. A calculation method for the rigid body’ s rotational inertia through the trifilar pendulum testing was developed by considering the influence of cycloid mass based on the law of energy conservation. A virtual prototype model of the trifilar pendulum testing system was established for the proposed method validation. The calculation errors were compared between the proposed method and the one without considering the cycloid mass effects. The effects of the cycloid’ s linear density and length on the testing error for the rotational inertia were further investigated. It is shown that using the proposed calculation method,the testing error can be reduced by about 50% compared with that without considering the cycloid mass through the virtual testing, and the testing accuracy of the trifilar pendulum method can be significantly improved.