To provide a labor-saving method for changing the production grade of polymer by a computer system capable of quickly, surely and efficiently performing the complicate changing operation without necessitating a skilled operator.

The method for changing the production grade of a polymer by a computer system comprises the use of data bases stored in the system and consisting of a production formulation database (1) for each production grade, a restriction condition database (2) in the change of the grade and a logical database (3) for the change of the operation items in the change of the grade, the preparation of a changing operation pattern (4) and an operation result forecasting pattern (5) by a dynamic simulator based on the above data bases and the automatic change of the polymer grade in the real plant using the prepared changing operation pattern.