The ‘Harness Model’ in the CAL3D occupant simulation program is improved to incorporate a reference point generation scheme and a new belt slip algorithm. The reference point generation scheme results in more accurate belt geometry and alleviates the need to manually specify the reference points coordinates on the occupant body. The new belt slip algorithm balances the belt force and the friction force at each reference point in a successive fashion. This approach gives satisfactory results and does not have the convergence problem found in the old slip algorithm. These new enhancements were used to developed a model to simulate the Hybrid III dummy response in a barrier test. Good correlation was obtained between the model response and the test results. Parametric studies indicated that the shoulder belt stiffness has a significant effect on the head motion, the abdomen deformation, and the peak shoulder belt force. On the other hand, the lap belt stiffness affects the abdomen deformation, the femur loads, and the peak lap belt force. The knee bolster stiffness affects the pelvis acceleration and the femur loads. The guide loop location affects the head motion and the peak shoulder belt force.