Precise pulsewidth measurement of petawatt laser with high energy is important to diagnosis of the focal intensity of the off-axis parabolic mirror and analysis of the gratings′ damage threshold. Two sources of error are discussed in this article, namely, the beam pointing and the periodic modulation of the near field distribution. The results show that when the stability of mirrors is within 5 μrad, the leam pointing error is 0.03%. The error decreases when the modulation period of near field increases. The error increases when the modulation depth increases. Besides, adopting mirror structure can reduce the measurement error caused by the near-field defects. When the period modulation depth of the near field is 1.5 and the modulation period is greater than 10, the comprehensive error is less than 20% and the minimum can be reduced to 10%. Total error of mirror structure is overall below 15% and the minimum can be reduced to 0. In the petawatt laser pulse width measurement experiment, the effect and improvement of the modulation of the near field on pulse width measurement is demonstrated.