Abstract: The key to application of random field theory to reliability analysis is how to combine the parameters obtained by the random theory,especially the combination of the reduction function of variance with the reliability theory.It is very important to correctly choose the reduction function of variance and to reasonably apply it to the reliability analysis.The problem often neglected by people was studied based on the random field theory and the principle for determination of reduction function of variance was presented.It was proved with an example that the principle is reasonable and reliable in the engineering practices.