In order to study the mechanical properties of Magnetorheological Damper (MRD) under quasi-static action, a four-coil shear valve MRD was fabricated. A quasi-static test was carried on the MRD by using the triangular displacement loading system under different currents and displacements. In the test, the variation of the mechanical properties of MRD with the changes of current and displacement was studied, and the causes of the damping force fluctuation phenomenon were analyzed. Based on the test results, a modified mechanical model under quasi-static loading was proposed, the parameters of the model were identified, and the mechanical model was verified. The results show that the damping force of MRD fluctuates remarkably during the quasi-static test process. The damping force decreases slightly with the increase of displacement amplitude when there is no current, and increases slightly with the increase of displacement amplitude when there is current. The damping force increases obviously with the increase of current, and when the current exceeds 1.5A, MRD reaches magnetic saturation and the increase of damping force decreases obviously. The mechanical model proposed in this paper can well describe the variation of damping force with the change of current and displacement under quasi-static force.