In order to study the equivalent relation between the maximum discharge currents (〖WTBX〗I〖WTBZ〗max)of two types of SPDs (ClassⅡ test and ClassⅠ test), the paper uses the method of taking 〖WTBX〗I〖WTBZ〗max 〖KG*5〗(8/20 μs) and 〖WTBX〗I〖WTBZ〗imp〖KG*5〗(10/35 μs) current waves as oblique waves, and calculates the 〖WTBX〗I〖WTBZ〗max〖KG*5〗(8/20 μs) current wave energy exerted on the ClassⅡ test SPD and 〖WTBX〗I〖WTBZ〗imp〖KG*5〗(10/350 μs) current wave energy exerted on the ClassⅠtest SPD, equals the energy of the two, then calculates the ratio of 〖WTBX〗I〖WTBZ〗max〖KG*5〗(8/20 μs)and 〖WTBX〗I〖WTBZ〗imp〖KG*5〗(10/350 μs), both of which should be satisfied. It is concluded through the calculation that when the voltage protection level of ClassⅡ test SPD is equal to that of ClassⅠtest SPD and the ClassⅡ test SPD maximum discharge current is 2156 times as much as that of ClassⅠtest SPD, the both bear the same energy.