A method and terminal for OTA upgrading are described. In the method, a system image file of a terminal to be upgraded is saved in the form of data blocks. The method includes: acquiring and parsing an OTA upgrade package to obtain respective differential upgrade files for the system image file; reading data blocks corresponding to respective differential upgrade files in the system image file, and performing an upgrading operation on corresponding data blocks to upgrade the system image file. Split system image files are used rather than files in a file system as data for differential comparison. Thus metadata in the file system are also included in differential data packet. After the upgrading, the image in the terminal is fully consistent with the image in a production process and does not conflict with a system check mechanism.