The resolution and observing width of optical remote sensing camera can be enhanced by increasing the detector length of focal plane. Optical butting is used to increase the imaging length owing to its simple structure, low cost and the simultaneously-imaging detectors. But butting process is influenced by many factors, which result in imaging gaps, so overlapping pixels between adjacent detectors is the key. In this paper, the factors causing imaging gap are analyzed, and calculation of overlapping pixel number is given out based on the analysis. Firstly an optical-butting focal plane system is built on optical imaging principles. Then the factors causing imaging gap are listed and analyzed, under the consideration of telecentric and non-telecentric optical system. Based on the imaging gap analysis, a formula for overlapping pixel number calculation is obtained, with the MTF assurance in vignetting districts resulted from optical butting. Finally a calculation example of a camera is given.