As a typical multi-body dynamics system，the design of principle parameters for a soft yoke single point mooring system（SPM） is a tough problem for its so many principle parameters. We use the modified Morris filter method with the perturbation analysis technology（fixed step size 5%，and perturbation range ±25%） to study the maximum horizontal mooring force and displacement on which much attention is focused during the system design. Considering the steady force，high-frequency force and second-order wave drift force of moored body，we establish an ideal mechanism model of SPM to study the local sensitivity analysis on the parameters，so that can strengthen the identification of the model parameters and uncertainty，and provide some useful decision-making basis when determining the principles of SPM. The results show that for SPM with 13 DOFs and 8 principles，the descending order of sensitivity of parameters related to the maximum mooring force and displacement in the model is the length of the yoke，length of mooring leg，and ballast force，so these variables should be given corresponding priority in the design process.