Impedance character as well as relevant impedance criterion is important in thesmall-signal stability analysis of multi-sgage dc distributed power systems, where subsystems are integrated. At the same time, we usually approximately simplify a tightly controlled load-converter as constant power load. In this paper, we focus on the impedance character and impedance criterion in two-stage dc distributed system, which is in fact criterion in two-stage dc distributed system, which is in fact inter-dependent dynamical system with both interaction loop and many individual regulation loops. We verify that minor loop gain method can be directly extended to inter-dependent subsystems, while original impedance criterion cannot. Furthermore, we develop a substitutional stability analysis. With this substitutional impedance criterion, we discuss the influence of regulation loop gain, load resistance, input voltage and various duty-cycle ration on system small-signal stability. In additon, we analyze the impedance shift process of the imput impedance of load-converter and build the relationship between it and rgeulation loop gain. Negative rgsistor approximation of load-converter is only regarded reasonable when high regulation-loop gain is assumed and low-frequeney character of load-converter is considered. Simulation results illustrate the validity of the substitutional impedance criterion.