As an important part of the novel power system, the renewable energy converging and boosting power grid has the phenomenon of cross and mutual supply, which leads to the inaccurate calculation of the existing line loss. In this paper, an accurate calculation method of line loss is proposed for the renewable energy converging and boosting power grid with cross and mutual supply. The measurement data of the renewable energy pooled booster grid is preprocessed. By analyzing the relationship between the line loss and the measurement data, the measurement data are classified according to the time scale and power flow direction based on the photovoltaic output characteristics and the power direction of the renewable energy pooled booster grid. On this basis, a new calculation method of line loss considering cross and mutual supply is proposed according to the time scale. The calculation results of the existing line loss measurement method and the new method are compared according to the representative day measurement data. The results show that the new calculation method considering cross and mutual supply can reflect the line loss more accurately, and provide a new method for the accurate measurement of line loss in the high proportion of renewable energy distribution network in Northwest China.