For innovative design of existing products, in order to fully consider the different types of user needs information, the extension innovation method was used to study its formalization and process, and the existing product innovation design method based on user needs and functional integration was proposed. The user requirement affair-elements were established and compared with function affair-elements of existing product, if the types were identical, the original system was analyzed through functional analysis, then the problem function was found and the original system function structure model was improved by using basic element and extension transformation; if the types were different, the matter-elements of functional action were obtained through the mapping relationship between user requirement affair-elements and function affair-elements; the feature and magnitude of the structural relation element were obtained according to the mapping relationship between the functional matter element and the structural relation element, and a new functional system model was established; the function of the two models was integrated and the functional configuration and design scheme of the new product were obtained. The feasibility and effectiveness of this method were verified through the innovative design of existing photovoltaic panel cleaning equipment.