A system and method for downloading software is provided. When software is required to be downloaded to the mobile terminal, a software downloading tool on the computer terminal establishes a connection with the mobile terminal via a preloader port of the mobile terminal and sends a download agent to the mobile terminal. A preloader program of the mobile terminal checks whether the download agent is signed and encrypted by a private key matched with an RSA public key in the preloader program, and if yes, the mobile terminal utilizes the DA download agent to download the software. The method can effectively prevent illegal tools from having communication capability with the mobile phone by USB connection for data deletion or tampering, and reduce the possibility that a hacker damages “limiting function” of the mobile phone.