The normal distribution assumption of measured results in the Lamb waves-based damage detection work was studied. Through the analysis of the results of numerical simulations, it was proved that the distribution of the arriving time measured by using threshold method was still not a topical normal distribution even under ideal Gaussian white noise. Therefore, the new algorithm of non-parametric statistics was introduced to give the accurate estimate about the distribution. The damage detection numerical result shows that the using of the correct probability density function of the measurement results have a direct impact on the damage location detection accuracy.