The invention discloses a modified direct-current power flow algorithm suitable for online analysis of a distribution network and belongs to the field of power flow analysis for power systems. The algorithm is applicable to the fields such as online analysis of the distribution network and real-time optimal control. The algorithm includes solving active and reactive power flows of branches of the distributed network by forward sweeping of node injection powder from the tail end of the distribution network to a root node; solving voltage phase angles of nodes of the distributed network by back substitution from the root node of the distributed network to a distribution terminal; acquiring linear equations of node voltage amplitudes and node injection powers by omitting deviation second-order terms of node voltage amplitudes of the distributed network, and solving the linear equations to obtain node voltage amplitudes. The algorithm has the advantages that the advantages of the standard direct-current power flow method are retained, no hypothesis is made to line parameters, the algorithm is suitable for radial power grids of any parameters, both the node voltage amplitudes and the node voltage phase angles can be computed, and the solving precision and the value stability are greatly improved as compared with those acquired by the standard direct-current power flow algorithm.