This paper analyses the mechanism of extra earning by which the contractors make unbalance bid by utilizing engineering change, reaches two key nodes, and describes the process of engineering change with the help of the idea of triggering.On the basis of these, it constructs a FSD model which contains the forecast of engineering change, the simulation of the appearance of the expected engineering change and the design of the Bidding strategy.This model can be used as theoretical basic and provide contractors a standardized operation process of unbalanced bid.