In this study, we investigated the effect of moisture content on the internal friction angle of a binary granular straw-coal system using a Freeman FT4 powder rheometer. The results show that, as the moisture content increases, the internal friction angle decreases at first and then increases to reach a minimum value at a moisture content ranging from 8% to 10%. This irregular trend, associated with the preferential adhesion of finely pulverized coal particles to certain regions of the straw, is more complicated than the conventional understanding that the internal friction angle increases with the moisture content. An empirical model is proposed to describe the quantitative relationship between the internal friction angle and the straw moisture content, and this model fits well with the experimental data.