The invention relates to a power distribution network loss assessment method based on a least square fitting method under non-sound information. Firstly, power distribution network topological structure parameters and the data measured by a transformer substation SCADA system are collected, bad data distinguishing and detection are conducted on the data measured by a transformer substation SCADA system, missing data and larger-error data in the data measured by a transformer substation SCADA system are processed by adopting data interpolation and the least square fitting method, load flow calculation is conducted on the data processed by adopting data interpolation and the least square fitting method, finally power distribution network operating parameters obtained through load flow calculation use feeder lines as basic units, the losses of the feeder lines are calculated according to the active powers of buses on the feeder lines, and accordingly the loss of a whole power distribution network is calculated. The power distribution network loss assessment method overcomes the shortcomings of large calculation errors and large calculation work amount of an existing power distribution network loss calculation method.