The invention discloses a business expansion analytical prediction system and method based on seasonal adjustment. The method comprises: firstly carrying out seasonal adjustment on the electric quantities of clients to eliminate the influence of seasonal fluctuation on an electric quantity growth trend, seeking for an inherent law of a time sequence, screening representative typical customers, and obtaining a release rule of a newly installed capacity of a client group by virtue of a principal component analysis method; and secondarily carrying out seasonal adjustment on expanded capacity and power sale quantity, respectively, and finding the month with the strongest degree of association of a trend cycle item by virtue of a gray relative analysis method to predict the power sale quantity. The system and method disclosed by the invention have the benefits that by considering that the electricity consumption of typical clients is affected by seasonal factors, the complexity of seasonal fluctuation covers the objective law in the economy development process and prediction of sequence becomes complex, the seasonable factors and other random factors are eliminated from an original sequence, so that the selected typical clients can represent the electric quantity change conditions of the client group in a real sense.