The invention discloses a phase-locus-analysis-based practical dynamic security domain construction method. The method comprises: step one, on the basis of practical power system data, a corresponding dynamic model is established, and a fault line and a fault state that correspond to a dynamic safety domain needed to be solved are determined; step two, a power angle-angular speed-based phase locus is obtained by transient simulation of an initial operating point of a system, a transient stability situation and a power angle leading cluster of the system are determined by combining a power angle curve, and then the power angle leading cluster is selected as a key cluster; step three, outputs of respective generators are changed to solve leading instability critical points of different generators and phase locus functions of the leading instability critical points are calculated and obtained; step four, at the leading instability critical points, small disturbance outputs deltaP of the generators are changed by means of time domain simulation and thus phase locus sensitivity is calculated; and step five, a phase-locus-based practical dynamic safety domain is constructed by combining the upper limit and the lower limit of the generator output and the output constraint of a balancing machine.