To overcome the disadvantages of single non-metric camera with small format and short baseline, a composite camera was developed with the combination of four individual non-metric digital cameras. The large frame composite camera was able to meet low altitude photogrammetric applications with high accuracy and efficiency. Since small movements of one camera against the other cannot be avoided due to a simple and low-cost structure, a relative self-calibration method was proposed to make up error derived from deformation of mechanical structure based on tie points in the overlapping areas of each two sub images. Analysis of factors influencing mapping accuracy was given based on the mosaic model used to project the images of the individual cameras to a single virtual image. Experiments indicate that the relative self-calibration method corrects small deformation of mechanical structure feasibly and the mosaic images meet the needed accuracy.