Aiming at the problems in the existing design research of variable-degree-of-freedom mechanisms that mainly focused on topological synthesis and lacked of systematic research on its design process, the evolutionary theory was introduced to complete the design of variable-degree-of-freedom mechanisms. Firstly, according to the design requirements and predefined tree diagram structure, a “source” tree diagram model was established;secondly, evolution operations such as reproduction，crossing and mutation were used to expand the mechanism to obtain the tree diagram model of underdrive mechanism;finally, the tree model graph was transformed into mechanism movement diagram. The mechanism that met the axiom design criteria and motion requirements was obtained through screening and evaluation. Based on this, a prototype system for intelligent design of variable degree of freedom mechanism was constructed. The system included four modules of requirements analysis, evolutionary calculation, evaluation and screening, and database, which realized intelligent design. The realization process and programming steps of the system were used to illustrate the operation of each module. Taking the driving mechanism of picking machine as an example, the validity and rationality of the prototype system was verified.