Abstract: Based on the method of standard value of geotechnical parameters in Norm for investigation of geotechnical engineering, the simplified formulas of correction factor in confidence level of 0.95 and 0.90 are discussed, and applied to the calculation of standard values of mechanical parameters in coal exploration. The result shows that the standard values calculated by the said two formulae were close to that by the theortical look-up table method in small number of test samples, and one-sided 90 percent confidence lower limit obtained by the standard normal distribution coincides with one-sided 95 percent confidence lower limit obtained by the normal distribution with the circumstances of sample size of greater than 50.