A λ-formed four-bar linkage to realize approximate straight tract was designed by using analytic method. A general synthesis model was established, and some key parameters in the equations were solved. By varying the design parameters, an infinite number of possible mechanical solutions can be represented, from which it is difficult for designers to select a satisfactory solution. To avoid aimlessness in choosing design parameters, the variable feasible areas could be computed and displayed with 3-D graph by applying numerical method and graphics technology. The property parameters, such as straight-line deviate tolerance, maximal link-length, mechanism type, have been compared and analyzed. The results of synthesis examples and property analysis verified the correctness and effectiveness of the proposed formulas and method.