According to the branch number of the equivalent circuit difficult to be determined, this paper uses the mathematical model of return voltage, and analyses the relationship of the recovery voltage’s mathematical expressions between branch’s parameters. Then it presents decomposition method of the return voltage for separating each sub curve of return voltage which polarization branch represented, thus determines the number of the equivalent circuit’s branches. It shows that the number of polarization branches of the equivalent circuit can be determined accurately and the aging condition of the transformer insulation is reflected more accurately by decomposing return voltage curve. Moreover, the more serious of the transformer insulation is, the more numbers of return voltage subcurve and polarization branches of the equivalent circuit are. This work is supported by Youth Fund of National Natural Science Foundation of China (No. 51407151) and Natural Science Foundation of Fujian Province (No. 2016J01747).