Based on the rule of Drucker-Prager, this paper sets up orthogonal test, taking incompetent surrounding rock's mechanical parameters as predisposing causes and analyzes the change of displacement field of tunnel. It takes the value of key point's displacement as target to compare and study such mechanical parameters' contribution degree. At last neural net math model is built to describe nonlinear relationship, appropriate surrounding rock's parameters being taken in then output being relatively accurate tunnel displacement field.