The invention discloses an on-line calculation system and method for the theoretical line loss of a power grid, and the method comprises the steps: reading topological structure data from a GIS information system in a mode of interface service, and storing the topological structure data in a line loss calculation database; reading the parameters of operation equipment from a PMS system in a mode of interface service, and storing the parameters in the line loss calculation database; reading operation data from a Open3000 system and a power utilization information collection system in a mode of interface service, and storing the operation data in the line loss calculation database; achieving the querying, reading and verifying of a graphic structure, equipment parameters and operation data stored in the line loss calculation database; employing the Newton-Raphson power flow algorithm to calculate the theoretical line loss, and displaying calculation results in a mode of graphics. The system and method effectively improve the accuracy of line loss calculation results and work efficiency, and alleviates the labor of a calculation worker in searching and processing data. The calculation results are in real time, are accurate and comprehensive, and correctly reflect the condition of power loss in a transmission process of power.