In a position measurement system based on a phase grating, the grating undergoes asymmetric deformation during processing and post-processing, resulting in a position measurement error. Therefore, this paper established a theoretical diffraction field model of an asymmetric grating and analyzed the influence of grating asymmetry on position measurement accuracy. On this basis, it proposed a weight optimization method with multi-diffraction orders according to the different sensitivity of the diffraction orders to the grating asymmetry. The method is expected to correct the measurement error introduced by the asymmetrical grating deformation. The experimental results show that when the center groove depth of the grating is 1/4 of the incident wavelength, the influence of top tilt asymmetry and bottom tilt asymmetry on the measurement accuracy can be neglected. When the duty cycle is 0.5, the effect of sidewall asymmetry can also be neglected. The position error introduced by asymmetry can be kept within 0.05 nm by the weight optimization method with multi-diffraction orders.