Shear strength parameters are important factors to evaluate the stability and economical efficiency of a geotechnical project. The most effective and mainly used method to estimate the shear strength parameters is linear regression. When this method is applied to calculate the shear strength parameters of core clay in Xiaolangdi dam with 320 triaxial tests

the results have shown that the standard deviation and coefficient of variation decrease when the number of sample increases. The theory analysis also revealed this trend. So the conventional linear regression method cannot reflect the real variation character of shear strength parameters. A modified weighting linear regression method whose results are stable and do not vary with the sample size

is developed based on the probability theory. The modified method is applied to calculate the shear strength parameters of core clay of Xiaolangdi dam; and the results are compared with the outcome obtained from the moment method

which verifies the validity and practicability of the new method.