In order to eliminate the imaging gap of focal plane based on optical butting, an error analysis-based algorithm of overlapping pixels was proposed. Firstly, an imaging model for focal plane based on optical butting is established according to the imaging principle of camera. Then, the reason causing imaging gaps is analyzed which combined with telecentric and non-telecentric optical system. Combined with the decrease of MTF in the vignetting distortion caused by the reflector, the formula for computing the numbers of overlapping pixels between adjacent CCDs is obtained. Finally, an example is given. Experimental results show that the proposed algorithm can eliminate the imaging gap.