Abstract: Air cathode instead of tin cathode is used to synthesize electrochemically sodium perborate in Na_2CO_3-Na_2B_4O_7 medium.The electrolysis result indicates that the cathode current efficiency is 65%～54%,the corresponding cathode current density being 30～50mA·cm~(-2).X-ray diffraction indicates that the crystal product is NaBO_2·H_2O_2·3H_2O.Constant current chronopo-tentiometry is used to determine the relation between time and potential in Na_2B_4O_7-H_2O_2 medium and in Na_2CO_3-H_2O_2 medium.The rate constant for the complexation of hydrogen peroxide and borate ion and the dissociation rate constant of perborate ion are 7.21×10~3 L·s-l·mol~(-1)and 2.14×10~2s~(-l)respectively,which are calculated from the relati on between iτ~(1/2)and i in Na_2B_4O_7-H_2O_2 medium.The iτ~(1/2)is independent of i in Na_2CO_3-H_2O_2 medium,which indicates that the dissociation rate of percarbonate ion is very fast.