The utility model discloses a transformer power characteristic live test appearance, through simultaneously to just in the monitoring of sampling of the signal of the input of operating transformer and output, obtain the body total lossess of transformer, signal to the output carries out a lot of sampling again, because the load in the electric wire netting is changing at any time, the voltage of output, the electric current also can the in certain extent fluctuation, select the corresponding current signal of close voltage signal in sampled signal, the no -load loss of transformer in these two sets of collection signals is the same, consequently, the difference of body total lossess is because load loss's difference causes, it is the load loss's that causes of the change of current signal change, consequently the relation through load loss and electric current can obtain the load loss of transformer, and the body total lossess of transformer subtract the load loss of transformer, can obtain the no -load loss of transformer.