Functional periodicity plays an important role in the process of product automatic innovation for ensuring stable systems. But there is no method of establishing Functional Periodicity. In order to establish the functional periodicity of a new product, an existed product called the goal product is firstly chosen, if it is similar to the new product on aspects of function, effect or structure. Subsequently, difference between the new product and the goal product is identified by establishing the mapping relationship between the functional requirements and design parameters. Using the tools from TRIZ, the function model of the new product is formulated based on the above-mentioned mapping relationship. Afterwards, the system functional periodicity of the new product is proposed in form of TRIZ function model by combining the results of proposed function model and the relationship among functions. Finally, the feasibility of the proposed method is verified with a specific design case.