Multi-kilojoule petawatt lasers using chirped-pulse amplification need large-scale compression gratings. Tiled-grating approach has been adopted in several systems to meet the size requirements, grating tiles need to be precisely phased to ensure a transform-limited focal spot when focusing high-energy laser pulses in the target. Piston error of segmented gratings induced the focal spot to split when it arrived at multiple of the half wavelength. Alignment method using ultra-short pulse capable of controlling the absolute piston phase error within one half wavelength is proposed.