The expanding of the pulse duration due to the dispersion of the beam splitter adopted in normal single-shot second-order auto-correlator (SSA) was calculated. It is shown that, for ultra-short pulses, this kind of pulse-expanding will introduce imprecision of the result (e.g., for a pulse with the duration of about 3 fs, the expanding of a 100 μm thick beam-splitter of K9 glass is 10%). Based on these results, a kind of total-reflection single-shot auto-correlator is proposed. Via changing the beam splitting method from wave-front splitting to wave-surface splitting, the dispersion of the beam-splitter is avoided. By adopting this kind of SSA, the accuracy of the measurements for ultrashort pulses is improved. The calculated results show that the accuracy is about 3.6 fs.