The present invention provides a power transmission capability evaluation method considering node power injection and power grid topology. The method comprises the following steps of: A. establishing an index named a spatial characteristic length ratio (SCLR) based on graph theory knowledge and a flow tracing technology to represent the transmission capability of a power grid; B. calculating an average value of path impedance multiplied by path power of a minimum weighted path among all node pairs in a normal network to obtain a numerator of the SCLR; C. calculating an average value of path impedance multiplied by path power of a minimum weighted path among all node pairs in a minimum spanning tree network to obtain a denominator of the SCLR; and D. integrating results in the step B and the step C to obtain the power transmission capability index SCLR. By using the method provided by the present invention, the changed power transmission capability when injection power changes or a network topology structure changes in the power grid can be effectively evaluated; the method has the characteristics of easiness for programming implementation, clear physical significance, strong adaptability and the like; and the method provides a basis for a power grid planner to propose an optimal power grid upgrading scheme.