The embodiment of the invention discloses a file decryption method and device and a client side. According to the embodiment, the method includes the steps that the client side receives JS code filessent by a server, obtains the sequence of the JS code files sent by the server to obtain a first sequence, generates a shared secret key according to the identification information of the client sideitself and the JS code files of the first sequence and decrypts the JS code files according to the first sequence according to the shared secret key. In the embodiment, the JS code files are encryptedaccording to the first sequence by using the shared secret key, so that the safety of the JS code files is effectively ensured, and the files are prevented from being tampered or attacked.