This paper describes the single tooth test for studying the rock breaking process of roller-cone bits in mud rock under simulated downhole pressure conditions (from 0 to 5000m). The tests are carried out under the laboratory condition using deep-well rock breaking tester. The results indicate that the penetration rate slows down with depth is attributed to increasing rock hardness,increasing cutter forces,increasing specific energy and decreasing volume of broken rock with well depth increasing. Therefore,it is necessary to determine the cutter shape and the cutteroplacement pattern in order to fit deep formation drilling and increase the rate of penetration.