Abstract: Based on the theory of material's solidifying transformation and CCT (continuous cooling transformation) curves of steel sections, an idea of coolant passage control was introduced into the plate cooling process with an exemplification given. A flexible coolant passage control system was then designed and developed by way of key units set dynamically, flexible cooling strategy and cooling water flow rate controlled to affect grain size. The control system was used to simulate the coolant passages in the industrial tests for the 400 MPa super steel plates, and the results were analyzed for the inherent causes of different microstructures/properties of the plates. Thus, a powerful technology as the ways and means was provided for the optimization of cooling process especially the development of new steel grades.