A file downloading method, characterized in that, comprising: acquiring, by a network management system, an original file; encrypting, by the network management system, the original file to obtain an encrypted file and encryption parameters corresponding to the encrypted file; sending, by the network management system, the encrypted file to a device system via a file transfer link and sending the encryption parameters to the device system via a security link; and after the encrypted file and the encryption parameters are acquired, decrypting, by the device system, the encrypted file according to the encryption parameters to obtain the original file, wherein the file transfer link is a File Transfer Protocol, FTP, link via which the network management system transfers files between itself and the device system, and the security link is a Transmission Control Protocol, TCP, link; wherein the encrypting the original file to obtain an encrypted file and encryption parameters corresponding to the encrypted file, comprises: dividing, by the network management system, the acquired original file into N fragments according to a preset division rule, wherein N is an integer greater than or equal to 1; and encrypting and recombining, by the network management system, the N fragments to obtain the encrypted file as well as a set of fragment flags for recording a position of a head of each encrypted fragment in the encrypted file and a set of keys for recording a key of each fragment, wherein the encryption parameters comprise the set of fragment flags and the set of keys; wherein the preset division rule comprises randomly dividing the original file to obtain N fragments each of which is longer than or as long as a smallest encrypted data block. acquiring, by a network management system, an original file; encrypting, by the network management system, the original file to obtain an encrypted file and encryption parameters corresponding to the encrypted file; sending, by the network management system, the encrypted file to a device system via a file transfer link and sending the encryption parameters to the device system via a security link; and after the encrypted file and the encryption parameters are acquired, decrypting, by the device system, the encrypted file according to the encryption parameters to obtain the original file, wherein the file transfer link is a File Transfer Protocol, FTP, link via which the network management system transfers files between itself and the device system, and the security link is a Transmission Control Protocol, TCP, link; wherein the encrypting the original file to obtain an encrypted file and encryption parameters corresponding to the encrypted file, comprises: dividing, by the network management system, the acquired original file into N fragments according to a preset division rule, wherein N is an integer greater than or equal to 1; and encrypting and recombining, by the network management system, the N fragments to obtain the encrypted file as well as a set of fragment flags for recording a position of a head of each encrypted fragment in the encrypted file and a set of keys for recording a key of each fragment, dividing, by the network management system, the acquired original file into N fragments according to a preset division rule, wherein N is an integer greater than or equal to 1; and encrypting and recombining, by the network management system, the N fragments to obtain the encrypted file as well as a set of fragment flags for recording a position of a head of each encrypted fragment in the encrypted file and a set of keys for recording a key of each fragment, wherein the encryption parameters comprise the set of fragment flags and the set of keys; wherein the preset division rule comprises randomly dividing the original file to obtain N fragments each of which is longer than or as long as a smallest encrypted data block.