Co-phasing mosaic grating technology is an important method of manufacturing meter-sized echelle gratings, and the precision of piston error adjustment has a decisive influence on the mosaicking precision. To eliminate the periodic piston error in mosaic gratings and achieve the co-phasing mosaicking of large-size echelle gratings, the specific influence of the piston error on the point spread function of the mosaic gratings is analyzed theoretically. A piston error detection and adjustment method for grating mosaicking is proposed in light of the principle of interferometry. For this purpose, the Michelson interferometry system is utilized, and white light and two-wavelength measurement technologies are combined. Simulation calculation results of the piston change of interference fringes under different adjustment amounts are compared with the results of tests on the nano-translation stage. The calculation precision of the Fourier analysis algorithm is analyzed, and the precise piston error adjustment is achieved. The experimental results show that the piston error Δz of mosaic gratings achieved by the white light and two-wavelength measurement technologies is smaller than 6 nm, which means the proposed technology can meet the requirements of co-phasing detection of large-size mosaic gratings.