Small-xignal stability analysis methods based on an impendance criterion originat from the minor lop gain mehtod and are gradually utilized om twp-stage dc distributed power systems. In this paper, we conclude that the impedance criterion directly dependent on output impedance Z。(s) of the source subsystem and input impedance Zi(s) of the load subsystem is possible but gives and invorrect stability analysis for systems with a regulated source subsystem. Through introducing a mapped pure impedance of the load subsystems and the preliminary system, we develop a general three-step impedance criterion, with which a correct small-signal stability analysis can be guaranteed. regardless of the type of sorce subsystem. Furthermore, we introduce the application of the three-step impedance criterion in two small-signalstability analysis cases and utilize it in an example system to predict the sability shift process arising from the variation on the load resistance and input voltage value.