Two-factor online transaction safety authentication method and system

The invention provides a two-factor online transaction safety authentication system. In the system, a transaction server is used to provide an online transaction service; a mobile communication device is used to receive short messages; a client computer uses a first verification function to communicate with the transaction server, receives a first verification code through short messages to verify the transaction server and uses a second verification function to generate a second verification code, and then the transaction server uses the second verification function and the second verification code to verify the client computer so as to achieve the aim of mutual authentication.