The joint forces and moments applied to the joints in an air suspension system in truck are important input loads for lightweight and fatigue analysis of bushings, air spring brackets, torque arms and trailing arms. In order to derive a reliable solution of joint forces and moments, engineers will generally use Multi Body Dynamics (MBD) simulation software, like ADAMS, which can save time in product development cycle. Taking an air suspension in truck as a study example, a 2-dimensional quasi-static model of an air suspension, whose stiffness of air spring and bushing is nonlinear, is established in ADAMS environment. After that, simulations are performed at the typical and extreme working condition respectively, and the results are compared with another three cases. Case I assumes that the stiffness of air spring is linear but the stiffness of bushings, including torsion and radial stiffness, are nonlinear. Case II assumes that radial stiffness of bushings is linear, but torsion stiffness of bushings and the stiffness of air spring are nonlinear. Case III assumes that torsion stiffness of bushings is linear, but radial stiffness of bushings and stiffness of air spring are nonlinear. Case IV assumes that stiffness of all elastic components are nonlinear, as mentioned above. The comparisons with those cases mentioned show that the stiffness nonlinearity of air spring and bushing has a significant effect on the calculation for the joint forces and moments, but it is more efficient to set the torsion stiffness of bushing to a constant due to the characteristics of air suspension.