An image joining method for a scanner that scans and transfers image data to a terminal is provided. During scanning and data transfer, when the image data stored in a register is full, an image processor stops the scanning of a linear photodetector. Meanwhile, a matrix photodetector fetches and stores the image of a code strip. Then, the photodetectors are moved backward a distance larger than that being required to be accelerated forward when the scanner resumes scanning. After the data in the register being transferred and cleared, the linear photodetector resumes scanning at a normal moving speed. The code strip image is further fetched and compared with the stored one to get a joining point of scanned image data. By the joining point, the new and prior image data portions are joined and the scanning proceeds, and joined image data will not be overlapped or broken.