In the distribution network, three phase unbalanced operation of distribution transformer are widespread. Three-phase unbalanced operation will make additional loss of distribution transformer and lead to local overheating, this for the life of the transformer and load ability has a great influence. In this paper, equivalent circuit model is established for Dyn11 and Yyn0 two kinds of connection ways of distribution transformer, the loss of transformer is studied. Simulation and experimental results show that, the transformer which work under the situation that three-phase unbalanced makes loss increased dramatically, and with the increase of the unbalanced degree, the loss curve is a quadratic function. When the unbalanced degree is 50%, the wastage of the transformer will be doubled. In order to guarantee the safety and life of transformer, in the case of unbalanced runtime, needs to reduce its load factor.