Tiltrotor aircraft integrate the advantages of vertical takeoff and landing (VTOL) capability of helicopters with the forward speed and range of fixed-wing aircraft. However, rotor control systems experience significant load variations across flight modes, posing challenges for accurate load predictions. To address the challenge, this study develops a coupled multibody dynamic model of the rotor control system analyzing pitch link loads in multiple flight modes. The simulation results align well with flight test data, validating the model's reliability. Furthermore, accuracy improves when compared with CAMRAD computational results. At low airspeed in helicopter mode, pitch link loads exhibit periodic oscillation patterns. As forward speed increases, there is a noticeable rise in pitch link loads, with particularly significant amplification in high-frequency components. In conversion mode, the load amplitude decreases as the rotor tilting angle increases, accompanied by rapid attenuation of high-frequency components. In airplane mode, steady axial flow conditions yield predictable load trends. The validated framework provides critical insights into control load mechanisms in multiple flight modes.