Superconducting magnetic energy storage( SMES) is one of the application of superconductivity in the power grid.The magnet of 100 k J SMES described in this paper consists of 30 double pancakes wound by Bi2223 / Ag tapes. When working at design temperature 20 K and design current 200 A,the coils suffer different magnetic fields at different locations which results in different degradation of critical current of the tapes. Adequate current margin should be assured for the sake of safety and stability of the ...