Abstract: On the statistical calculation method of the geotechnical parameters in Norm for Investigation of Geotechnical Engineering (GB50021-2001), the paper introduced random field theory, got the average confidence limits under the confidence level of 1-α and used the standard value to express it. But because the random field theory has not yet been completely practical, a simplified formula for calculating the standard value was used, making the calculation formula consistent with the original norm. At the same time, in order to facilitate the use and avoid the misuse of student's distribution table, the norm carried out fitting and simplification of the calculation formula of the modified parameters. The paper, from the classical random variables theory, using statistical parameters in the estimation theory, discribed the standard value of the modified calculation formula, discussed the correctness of the simplified calculation formula, and puts forward the new and more reasonable fitting formula.