As an alternative for producing the meter-size diffraction grating demanded in many technical field, grating mosaic requires positional detection with accuracy of sub-microns. In this paper, we propose a positional detection method based on image processing of three far-field diffraction intensity patterns in two wavelengths. With a set of detailed adjustment steps deduced from theoretical analysis, we successfully detected and separated the two positional errors. Moreover, with the three patterns we enlarged the target range of coarse adjustment required for further fine adjustment in longitudinal position. We achieved positional detection sensitivity of less than 14 nm, and diagnosed the alignment with the far-field pattern in a third wavelength.