The calculated failure probability for foundation and slope is usually very large

even though the project is proved to be safe in reality. This phenomenon is resulted mainly from the method to estimate variance in practical project. Data of soil index is just property of a certain point rather than the space average characteristic of soil profile. In the process of reliability analysis of geotechnical engineering

the reduction function of variance is the key parameter to transit the point property into spatial average property. It’s the first time that improvement on the method to calculate the no correlation distance put forward by YAN Shu-wang et al. is made

which not only makes the calculation more convenient

but also is a breakthrough of theory. The procedure of calculation of reduction function by the improved method is analyzed

and general regularity of determining reduction function through calculation of no correlation distance is obtained.