Abstract: This paper deals with basic characteristics of incompetent rocks. According to experiment results of the expansibility and slaking durability, frost-thawing action, and mechanical properties of the weak rocks, the effect of time and environment (water content, temperature, stress in the earths curst) of the rock occurrence on the mechanical properties of the weak rocks are analysed. The variation regularities of the mechanical properties of different rock elements are summarized. The author propose the principle for determining strength of weak rocks.