The purpose of research on calculation of low voltage distribution network theoretical line loss is define a calculating method and through the method to calculate the low voltage distribution network theoretical line losses. The calculation of low voltage distribution network theoretical line losses is the foundation of the power system economic operation, the reactive power optimization and the grid technique modification. Through the theoretical loss calculation, the operation management department can undertake line loss analysis, thus they can adopt feasible loss reduction measures, and achieve maximum economic benefits. this paper puts forward through the power flow calculation system, to calculate the low-voltage distribution network users theoretical line loss. Matpower is a softwate package of matlab, which is for power flow and optimal power flow. Based on the relationship between power loss and power flow calculation, calculated the power flow using matpower package, and explained the role in the use of computing to slove the line losses. Through the IEEE (Institute of Electrical and Electronic Engineers) example of 14 nodes and some specific instances of low voltage distribution network calculation, proves that the method is feasible. The results also shows that matpower is suitable for calculating low voltage distribution network theoretical line loss.