An improved Freeman-Newell functional reasoning model is proposed for the conceptual design of manufacturing process control strategies. The structure in Freeman-Newell model is partitioned into determined subset and undetermined subset. Main framework of the structure is expressed by determined subset. Undetermined subset is presented to express the functions required from next design level. In this way variant design and multi level design reasoning can be supported. Based on the improved Freeman-Newell model, multi level conceptual design methodology for control strategies of manufacturing process is presented. Tracking control algorithm design for a nonlinear manufacturing process plant is provided as the example to verify the design reasoning ability of the improved model.