This paper designs an encryption algorithm for JPEG image files based on content-associated key technology. Based on the JPEG data stream, the algorithm divides the file data into a format data part such as metadata attributes, primary decoding tokens and secondary decoding tokens, and a compressed data part. The algorithm encryption process is divided into two stages, one is to analyze and locate the format data to form a key unit sequence, and the other is to randomly extract the compressed data to form a key unit sequence. Judging from the playback effect of the encrypted JPEG image file, the algorithm is effective.