The load act on the component is crucial to assess the durability performance. The boundary conditions of the test specimen under test is another factor to influence the fatigue lives by introducing unrealistic loads on the component of interest. Physical test is widely conducted in the laboratory. The fixture provide 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 paper. A simplified 7-DoFs (Degrees of Freedom) model is introduced to demonstrate the dynamic behavior of the vehicle. The influence on the internal load by the fixture was analyzed. Followed by a more detailed MBS (Multi-Body System) model to give an in-depth understanding of the phenomenon. A pseudo damage analysis was conducted to show the impact of the fixture on the durability performance of the axle test.