Cost management is the core of the management of commercial aircraft company , decomposing the cost of the aircraft is a key part of the management of commercial aircraft. The polynomial approximation is proposed to solve the cost allocation problem for commercial aircrafts. by making up polynomial function, the article gives each part cost of the commercial aircrafts in the proportion of the total cost, and provide the basis of the management of the aircraft. Considering the total cost which needs to be cut under certain, the article constructs a robin algorithm that gives the cut number of each part and the proportion of the cost reduction. The algorithm is proved to be true by studying case.