Abstract: To improve some deficiency of calculation of reliability index in response surface method (RSM), point estimation of probability moment deduced by Resonbluth was introduced into response surface method. Improved response surface method (IRSM) can take into account the departure degree for probability density curve of basic random variables when sampling. Calculation formula for the sample values of random variables was changed and the reliability degree index was calculated through probability moment in improved RSM. Some special points of real limit state curve surface was adopted when calculating reliability degree index in the IRSM. There are no need for linearization and fitting of approximate limit state function while calculating reliability degree index. Iterative and cumulate error of reliability degree index does not arise. Principle and calculation process of IRSM is much simpler than RSM. Stability reliability degree for surrounding rock of a large underground engineering was analyzed by ISRM and RSM and Monte-Carlo method respectively. Result calculated by Monte-Carlo was taken as a standard solution to show the accuracy of solution of IRSM. Comparison between the solutions of IRSM and RSM shows that result calculated by IRSM is more accurate than that of RSM. Accuracy of IRSM can meet engineering requirement. IRSM has both theoretical and practical value.