Abstract：Response surface method(RSM)is an efficient method to analyze the stability reliability degree for complicated structure，such as underground structure. However，general response surface function can not simulate a character of curve surface of limit state equation for a complicated structure，for the curve surface has more than one peak-value on its parameter space. Quadratic equation that contains mixed terms of random variables can overcome the above-mentioned defect. Therefore，it is presented that function with mixed terms of random variables is adopted to replace function without mixed terms of random variables in response surface method and test procedure，which are used to decide coefficients of function without mixed terms of random variables and calculate reliability index. It is improved to make it suitable to response surface function with mixed terms of random variables. The analytical expression that contains mixed terms of random variables can be decided by the improved general test procedure. Due to more than one peak value on curve surface of the analytical expression containing mixed terms of random variables，genetic algorithm is adopted to search a globe optimal solution，reliability index，and checking point in the improved test procedure. The improved test procedure and response surface method，considering quadratic equation that contains mixed terms of random variables as response surface function，is to used to research stability reliability degree for a large-scale underground rock mass structure. Other method，to take quadratic equation without mixed terms of random variables as response surface function，is adopted to analyze the reliability degree of the same structure too. Calculated results of Monte Carlo method are regarded as criterion，and it shows that the calculated result of the former method is more accurate than the later.
