To guarantee the security of the private keys of PKI users, a scheme of accessing and storing the private key is proposed in this paper. The trustee-boxes storing the private key are introduced to meet the roaming users accessing to the private key in the cross-regions; Making use of the characteristics of threshold technology, the divided private key slices are encrypted and stored into the n trustee-boxes, and the whole private key is restructured as long as k trustee-boxes are effective. By cross-certificating with the strengthen servers a user is verified as a legal user when it accesses its own private key. The secure performance of storing and accessing private keys is improved in the scheme. At the same time, the legal users are authenticated by system.