Abstract: It is very important for drilling engineering and geological staff to assess formation lithology and the working condition of bit. According to the conservation principle of energy and the rock-breaking mechanism of bit, the assessment model was set up for formation lithology and the working condition of bit. The model can predict the formation lithology, the apparent drilling strength, shearing strength of rock and the current working condition of bit by using various downhole or surface information about WOB, torque,drilling speed and revolution during drilling. And the real-time assessment software system for formation lithology and the working condition of bit was also developed. The initial onsite application of the software showed good results.