At the Shen Guang II (SGII) Petawatt Laser Facility, measurements of large-energy, single-shot laser pulses sometimes feature asymmetric autocorrelation signals, causing uncertainty in the measurement of compressed pulses. This study presents a method for defining and describing the asymmetry of autocorrelation signals. We discuss two sources of asymmetry: the nonuniform distribution of the near field excited by a beam, and the rotation of autocorrelator arms from the cylinder lens. The pulsewidth of an asymmetric autocorrelation signal is shorter than its real width. After updating the autocorrelator, the single-shot autocorrelator for the SGII petawatt laser exhibits a measurement uncertainty of below 12.3%. Recommendations on reducing asymmetry in large-energy, single-shot autocorrelation are discussed.