The electric power engineering cost management is an important content of power engineering construction management. In order to provide a quantitative analysis basis of project decision-making and project evaluation, a reasonable project cost interval that included 80% period samples was defined. An interval calculation method based on Chebyshev’s inequality was presented, which optimized the original data with using boxplot, to improve the credible degree of the calculation. The initial value of optimization iteration calculation was given based on Chebyshev’s inequality and the reasonable cost interval was calculated with using a one-dimensional optimization method called advance and retreat method. The calculation results have certain mathematics basis and can provide a reliable basis for the analysis of power grid project standardization construction.