Multi-channel focal plane registration of remote-sensing camera is the key step in camera assembling process. Registration accuracy determines quality of the image after channel fusion. The development of focal plane registration instrument for a nine-spectral double-channel camera was proposed in this article. A slit was illuminated by integrating sphere and imaged at infinity with collimator. The infinity slit was recorded by the nine-spectral camera, and became an enlarged vertical slit on camera focal plane. The four-spectral detector and five-spectral detector were on two different focal plane channels. By approaching the slit image centroid position on two different detectors, the two focal plane channel were aligned together in high precision. This instrument solved the problem of high registration accuracy of multi-channel camera. Sub-pixel registration accuracy could be reached by the instrument, assuring the camera image composite.