The co-phasing detection technology of segmented mirror is one of the key of the segmented process and maintaining the quality of mirror. The quantitative measurement piston error is not only the chief gauge of the adjustment of segmented mirror, but also the guarantee of the segmented telescope′s imaging performance and the segmented telescope′s angular resolution. In order to realize the detection of lager range and high-precision of piston errors of segmented mirror, the new two-wavelengths co-phasing detection algorithm was raised in this paper. The relation between the theoretical detection range and the values of two wavelengths was got and the relation between the max value of template interval and the values of two wavelengths was got based on circular diffraction and the two wavelengths detection theory. In two wavelengths algorithm, the gap error, eccentric error and camera noise between two half circular diffraction patterns could influence the detection range and accuracy of two wavelengths algorithm, how the errors of gap, eccentric and camera noise influence the detection range and accuracy by theory and simulation were analyzed and got. And the gap error less than 0.2r, the eccentric error less than 0.2r and the Signal to Noise Ratio (SNR) of camera noise more than 30 were got, the two wavelengths detection will not cause false detection. The study on error sprovided reference for the co-phasing experiment of segmented mirror in two wavelengths algorithm.