Fixed length code (FLC)-based image compression method and device are provided. The FLC-based image compression method for compressing a block containing multiple pixels includes the following steps: determining a first representative pixel and a second representative pixel from the pixels according to the pixel values of the pixels; generating, by interpolation, multiple interpolated pixels according to the first representative pixel and the second representative pixel; generating an index value for each of the pixels according to the first representative pixel, the second representative pixel, and the interpolated pixels; adjusting the first representative pixel and the second representative pixel to generate a first adjusted representative pixel and a second adjusted representative pixel; and using the index values and complete or quantized pixel values of the first adjusted representative pixel and the second adjusted representative pixel to represent the block.