In order to combine several segmented sub apertures into an equivalent large-aperture telescope according to the design goal, each sub aperture must be in optical co-phasing. In this paper, a co-phasing method based on a pyramid sensor is proposed. The sinusoidal relationship between the sensor signal and piston error is fitted through experimental calibration, and the piston error is inversely calculated. The experimental results show that the measured piston errors essentially conform to the linear relationship of the real values, the root mean square error is approximately 19.2 nm after fitting, and the measured value can objectively and accurately reflect the actual piston error. Based on this, the near co-phasing correction of a seven-aperture splicing mirror is conducted, and the resolution is improved by nearly six times after correction. Compared to traditional methods, the proposed method has the advantages of a simple structure, fast response speed, and high light energy utilization.