The invention relates to a method for using dynamic encrypted algorism to communicate network data, wherein it comprises: the server, base the rules, informs client about the time and method of dynamic encrypted algorism, and sends the algorism to the client; the client receives the dynamic encrypted algorism module, to check the client program and the legality of said algorism, and execute the module, to extract out the hidden encrypted and decrypt function, based on the real rules of algorism to encrypt and decode the data pack. The invention can effectively protect the data communication between server and client, to be used in instant communication software, FTP software, etc.