The mechanism of stress-corrosion cracking (SCC) of X70 steel in a weakly acidic model soil environment is investigated using slow strain rate method, voltammetry, optical microscopy, and scanning electron microscopy. New methodological approaches to the study of SCC under thin-film corrosion conditions are developed. The potential ranges, in which SCC mechanism of X70 steel changes, are determined. In the solution, for potentials more positive than −0.816 V (saturated silver-chloride electrode—SSCE), SCC mechanism is anodic dissolution (AD); for negatively than −0.949 V potentials, it is hydrogen embrittlement (HE), and in the range from −0.816 V to −0.949 V, SCC mechanism is mixed (AD+HE). For the model soil environment, a narrowing of the mixed-mechanism region to nearly 0.100 V is established compared to 0.133 V-region for the mixed mechanism in the solution. Moreover, shifting of the lower and upper limits of the mixed-mechanism potentials to more negative values is established, namely, to −1.070 V and −1.170 V, respectively. As determined, the coefficients of susceptibility to SCC, K_S, in the model soil environment are lower than in the solution. At the minimum protective potential of −0.750 V, the differences in the nature of SCC are insignificant; K_S is of 1.30 in the solution and 1.06 in the model soil environment. More consequential changes in SCC of X70 steel under thin-film corrosion influence are observed as the applied potentials are of −0.950 V and −1.050 V. As established, in the solution, SCC at these potentials occurs by hydrogen embrittlement mechanism, which correlates successfully with the brittle nature of specimen fracture and the K_S values of – 2.38 and 2.24. In the model soil environment, SCC occurs by the anodic-dissolution mechanism that is confirmed by ductile characteristics of the failure and K_S values of 1.39 and 1.18.