The invention discloses the theoretical line loss online accurate calculation system of actually-measured data. The system comprises the following processes of 1) a preparation process before an actual test; 2) a data actual measurement process; 3) a data arrangement process; 4) a line loss calculating process; and 5) result analysis. The system has great advantages in data graph acquisition, datacalculation method diversity, calculation result analysis, and calculation result report and summary report output. Aiming at a calculation result, line loss analysis and a loss-reduction auxiliary decision can be performed, and the reactive power optimization of a transmission and distribution network, transformer-substation economic operation, network loss sharing, harmonic negative sequence analysis, low-voltage area power supply position optimization, high energy consumption distribution transform replacement, voltage optimization suggesting, lead replacement, three-phase unbalance adjustment and other loss-reduction auxiliary decision functions are realized. According to the demand of a client, different calculation result reports are selected, and according to load actual measurement and the format of a line loss theory calculation analysis report template, the report is generated, which is convenient for filling in the report in a later period; and the accuracy of line loss calculating and work efficiency are effectively increased.