The present invention provides a method for protecting software based on network, which combines a client program that communicates with a server in C/S (or B/S) architecture with a key device, the client program authenticates a user using the key device for protecting software, the method includes the steps of: running the client program; authenticating the user using the key device by the client program; and continuing to run the client program with a server if the user has passed the authentication. In the prior art, the username and password are easy to intercept in transmission as plain text over network. In the method, the client program is combined with a key device. In addition, the complete client program cannot be executed without involving the server. Therefore, the strength of software protection is increased.