One of the key aspects of a recent rural grid upgrading and reform focuses on the grid which has already been reformed but still needs an upgrading due to the electricity demand growth. According to the current relevant technical standards， more than 70% of a rural grid should be reformed. Considering the actual situation of rural grid in Yunnan Province， a new concept for planning the rural low-voltage line is proposed， and a method is presented to determine whether the power supply length and conductor cross-section of a low-voltage line are rational with the constraints of allowable voltage and power loss. Also， based on the economic loading， a practical method and tabulation is proposed to make the low-voltage conductor cross-section suitable for the size of its power supply medium voltage distribution transformer. Actual application shows that， on the premise of meeting the electricity demand in the planning period， the grid percentage to be reformed is significantly reduced to almost below 20%， with the returns on investment greatly increased.