In order to realize automatic image mosaic with HJ-1 satellite images, this paper proposes a bisector seamline algorithm which is based on the geometric characters of the remote sensing images valid areas. Firstly, the valid area is used to replace primitive rectangle image to participate in image mosaic, the feathering seamline is automatically generated on valid area. Then the seamline is effectively removed by an improved feathering algorithm. Finally, through the mosaic experiment on a group of 9 remote sensing images, the experimental result indicates that the new algorithm is applicable for complex polygon overlap, easy to realize and has a good visual effect.