We develop an optimization method to approximate the region of feasible power injections in distribution networks. Based on the nonlinear Dist-Flow model of an alternatingcurrent (AC) network with a radial structure, we first formulate a power-injection feasibility problem considering voltage and current limits. The feasibility problem is then relaxed to a convex second-order cone program (SOCP). We utilize the strong dual problem of the SOCP to construct a convex polytopic approximation of the SOCP-relaxed feasible power injection region. We further develop a heuristic method to approximately remove the power injections that make the SOCP relaxation inexact, thus establishing an approximate polytope of solvable and safe power injections. Numerical results demonstrate a satisfactory balance reached by the proposed method between the accuracy of approximation and the simplicity of computation.