Electrochemical etching of silicon carbide (SiC) material has received increasing attention in recent years, due to its simple procedure, low cost and significance in exploring novel optoelectronic devices. In this paper, 4H-SiC substrates were electrochemically etched at a constant current of 1.0 A in an electrolyte made up of hydrofluoric acid and deionized water. The layering of SiC porous layer and the periodic fluctuation of the voltage were witnessed for the first time, and the layering phenomenon corresponded well to the voltage period. However, no such phenomenon was observed when the SiC substrates were anodic etched under the same conditions with magnet stirring. As a result, the periodic variation of voltage was hypothesized to be the cause of regular layering during the constant-current electrochemical etching. Electrochemical etching in potentiostatic mode was thus performed at different voltages. We found that the diameter of the SiC nanopores increased while the thickness of the sidewall decreased with the increasing voltage. Based on the experimental findings, a model of mass transport was proposed. The mass transport process would lead to the periodic changes in resistance, hence the periodic change in voltage. It explained the reason of the layering successfully. Furthermore, SiC substrates were also electrochemically etched at high and low currents to find that the threshold current existed for the occurrence of the layering. The energy dispersive X-ray spectroscopy (EDS) analysis showed that the composition of SiC porous layer remained unchanged comparing to pure SiC wafer, implying that the peeling-off SiC porous layer obtained by electrochemical etching can be directly adopted for use on devices requiring SiC porous structure.