The traditional direct iteration method has low computational efficiency, poor iterative performance and computational difficulties for the analysis of power frequency electric field (PFEF) of key devices within substation, especially involving multimedium.The preconditioning generalized minimal residual (GMRES) boundary element method (BEM) is proposed to enhance computational efficiency of three dimensional PFEF of key devices within substation, where it can’t be solved by the traditional direct iteration method. The basic principle and implementation steps are given. PFEF distribution of switch and break for 500kV substation switch field is presented to demonstrate the efficiency and accuracy of preconditioning GMRES BEM for studying large scale PFEF of key devices within substation. The proposed preconditioning GMRES BEM can not only improve convergence rate and residual convergence rate but also reduce computation cost without decreasing accuracy. Therefore, it is superior to direct iteration method and a suitable algorithm to solve the largescale systems arising from the PFEF of key devices within substation.