Abstract: This paper takes the horizontal cutting action of bit teeth on bottom hole rock into consideration, analyzes the varying law of bottom rock penetration coefficient and the shape of the initially penetrated broken pit, arrives at the bottom rock breaking process during repeated penetration by rotary drilling and the curve pattern of penetration depth of the load, and further reveals the rock breaking mechanism of roller bit in rotary drilling: with the increase of friction factor, the penetration coefficient of the rock before the blade is in linear decrease and the penetration coefficient of the rock after the blade is in linear increase after cutting. At initial penetration, the broken pit is asymmetric irregular shape, the volume of broken rock is smaller than when only vertical pushing-in effect is considered, and the load upon occurrence of shear breaking is low. After the rock before the blade is shear broken, for the rock after the blade, the overall load required by shear breaking decreases due to stress concentration, so shear breaking by multiple penetration into rock is easier at the presence of horizontal cutting effect. The frequency of shear breaking of bottom rock during repeated penetration is two times when only vertical pushing-in effect is considered, and the decrease rate of the load at shear breaking reduces.