Abstract Through analysis on the fundamental behaviour of soil-metal friction, a conclusion was drawn in this paper that a non-Newtonian fluid equation could be applied to describe the mechanics of soil metal-friction. Based on the Rate Process Theory, an expression of the friction stress as a function of sliding speed and normal pressure was established. This expression was verified with laboratory tests in the sliding speed range of 0.4–1.7 m/s and in various soil conditions. The experimental data also showed that the pattern of friction-soil water content curves at high sliding speed was similar to those at low sliding speed.