Abstract: For the suspension isolator constituted by trilinear pendulum, the Lagrangian equation with swing angle as generalized coordinate was established firstly at a particular direction. When the swing angle was a particular value, the three-dimensional graphic was projected onto a plane, other variables were converted to the functions of swinging angle by solving geometric relations, thus the horizontal natural frequency can be obtained. The impact to horizontal natural frequency from mechanism structure parameters were analyzed through adjusting the parameters in a certain range. A lot of simulation results were made by MATLAB. The results show that increasing supporting length can reduce the natural frequency, but increase mechanism height. It also shows that once mechanism stability is insured, increasing the ratio of fixed disk’s diameter to swing disk’s diameter and reducing the ratio of fixed disk and swing disk-s distance to supporting length can reduce the natural frequency, and not increase the height of the mechanism very much.