Due to bus bar function of single?ended injection for 10 kV main line, the end?terminal is prone to suffering low voltage problems. Transforming the main line voltage drop constraint into the outlet load constraint can provide great convenience for the safe operation and planning of power grid. Therefore, an estimation method of the first?end load security region boundary is proposed in this paper. Aiming at the numerous station loads along the line, a torque method for rapid estimation of the voltage drop across the main line basing on segmented voltage drop characteristics is proposed. Then the actual station load distribution is equivalent to the characteristic form of balanced distribution along the line with several heavy load points, which is more consistent with the actual situation. Based on the moment method, the relationship between the load at the head end of the line and the voltage drop is obtained, and finally the linearized equation of the safety zone boundary of the head load is obtained. The results of the calculation examples show that the voltage drop analysis and safety zone boundary analysis of the moment method satisfy the engineering accuracy requirements. Based on the boundary of the first?end load safety zone, the load margin of the 10 kV line can be obtained, which is convenient for load controlling and rolling distribution network planning, and has good engineering application value.