The AC distribution network has threephase unbalanced characteristics and can be connected with DC networks to form a AC/DC hybrid grid. This paper proposes a new type of πmodel for the AC/DC converter and establishes a set of power balance equations to describe threephase AC and DC sections. The AC and DC network is described by admittance matrix. The node admittance matrix of the whole network is then set up since the AC/DC converter can be included in the matrix by a π equivalent model. Combining the converter model and the network equations, a power flow calculating method including threephase AC line and DC line is proposed and verified with cases.