The invention discloses a power-off continuous transmission upgrading method and a terminal system thereof, which are suitable for a terminal system, and the method comprises the following steps: S1, downloading an upgrading packet, and caching a data stream into a buffer pool of a memory; s2S2, when the length of the currently cached data stream does not meet the size of a buffer pool, forcibly writing the cached data stream into a user-defined continuous transmission file, and updating and marking a file downloading starting point and a file storage starting point in real time by the continuous transmission file through a file recording pointer according to the writing of the data stream; and S3, if the power failure restart occurs, continuing to download the upgrade package according to the file download starting point, and continuing to store the data stream according to the file storage starting point. Through the implementation of the method and the device, the breakpoint resume is prevented from continuously downloading the upgrade package data after power failure and restarting, the upgrade problem appearing in the power failure and downloading process is solved, the upgrade function of the Android intelligent equipment is perfected, and better experience is brought to a user.