Load margin calculation is an effective method to evaluate the static voltage stability of distribution networks. According to the characteristics of radial distribution networks, this paper presents a novel method of load margin calculation based on the second-order cone programming approach. On the basis of the established load margin mathematical model for radial distribution networks with DG, the variable substitution method was used to turn this model into second-order cone programming standard form, so as to achieve the rapid calculation of load margin optimization model. Finally, the proposed method was applied to IEEE 33-nodes system and results are compared with traditional continuation power flow method. Numerical studies are validating the availability of the load margin optimization model as well as representing of the effectiveness of second-order cone programming approach.