The action of load on the component is crucial to evaluate the performance of durability. Another factor that affects fatigue life is the boundary conditions of the test specimen being tested by introducing unrealistic loads on the component of interest. The physical test is widely conducted in the laboratory. The fixture provides additional constraints on the test specimen as well as reaction forces to balance the test system. The characteristics of the fixture involved in the test is important to analysis and assess the test results. The impact of the reaction force of the fixture on the spindle coupled axle road simulation test is presented in this article. A simplified 7-DoF (Degrees of Freedom) model is introduced to demonstrate the dynamic behavior of the vehicle. The influence on the internal load by the fixture has been analyzed. Followed by a more detailed MBS (Multi-Body System) model to give a thorough understanding of the phenomenon. A pseudo damage analysis was conducted to show the impact of the fixture on the durability of the axle test.