The no － load test of main transformer often occurs in power plants and substations，due to differences in factory parameters of equipment，there is three － phase voltage unbalance in low voltage side of main transformer under no － load condition． Through vector analysis and equivalent circuit method，the reasons for three － phase voltage unbalance are analyzed，and the parameters of different units are measured． The data computation provides a reliable basis for three － phase voltage unbalance in low voltage side of main transformer under no － load condition．