It is particularly important to replace high-loss distribution transformers according to local conditions in the energy-saving transformation of distribution networks. Calculating distribution transformer losses under actual load conditions is one of the key points of transformer selection technology and economy. In low-voltage three-phase and four-wire system, unbalanced load often results in asymmetrical distribution and transformer three-phase current. The loss calculation model of transformer under three-phase unbalance condition is discussed at first, and then the current phase estimation method of transformer low-voltage side is put forward in view of the fact that the current phase of existing distribution transformer low-voltage side mining system data is missing. Thus, the loss calculation model based on distribution transformer current phase estimation under three-phase unbalance condition is established, which solves the problem that cannot be calculated due to missing phase data in actual engineering. Finally, the current phase estimation and loss calculation models of Yyn0 and Dyn11 commonly used transformers are verified by simulation and experiment respectively.