Information sharing can bring about better benefits to the supply chain.But enterprises in the supply chain have their own private information such as the cost information,quality information.So enterprises can seldom be fully sharing information.How to make joint decision considering the whole supply chain's optimization with private information protection? A model which a supplier and a retailer make the best ordering decision jointly is built in the paper,and homomorphic encryption is applied to solve the cooperative ordering decision with private information protection.The results show that enterprises with private information protection can get the results of complete information sharing by the secure computation protocol based on homomorphic encryption.