To make the high and extra-high voltage alternating current transmission line frequency electric field intensity theory calculation results conforms to the actual,the improvement of three-dimensional electric field intensity calculation formula is studied on the basis of two-dimensional and traditional three-dimensional.Through the existing high and extra-high voltage alternating current transmission lines electric field intensity of field measurement and compared with the theoretical calculation results.The results show that theoretical change tendency and calculation values are close to the measured values and change tendency.Different from traditional three-dimensional calculation method, the improved calculation formula is simple and easy to use,and the calculation accuracy as same as the traditional three-dimensional precision.At the same time ,in the guarantee the calculation accuracy has more practical application value.