The invention relates to a broadcast encryption method and system with a fixed private key and ciphertext length. A data owner encrypts data of the data owner by using a session key and uploads the encrypted data to a cloud storage server. A data subscription user submits identity information registration at a data owner and applies for access authorization. The data owner generates a private keyfor all the data subscribers and publishes system public parameters at the same time. The data owner uses the authorized data subscription user as a legal receiver and broadcasts and encrypts the authorized data subscription user into an encrypted session key through a public key. The legal data subscribers can successfully decrypt the broadcast ciphertext through their own private keys to obtainthe session key. the target data subscription user can continue to decrypt the encrypted data by using the session key so as to access the original data. According to the method, a pruning-merging algorithm is designed, the number of legal receivers needing to generate the private key is effectively reduced, and the method and system are suitable for achieving fine-grained access control.