Abstract: In Rumaila Oilfield, there are a small number of cores, so drilling bit optimization is mainly based on the experience of construction and design staff.To solve this problem, the correspondence relationship between rock drillability and log data of Rumaila area was established based on drilling rate equation by means of backstepping regression and data fitting.Then, the results were verified according to cutting fractal theory.Furthermore, the characteristic parameter which can reflect rock breaking difficulty best was determined and the statistical model on rock strength, hardness, drillability and their fractal dimensions was developed.Finally, ROP improvement test was carried out specifically on KM533X composite drilling bit on site according to the prediction results of rock drillability.It is shown that the average drilling footage and ROP of each bit is increased by 214.70% and 29.90%, respectively.And thus, the drilling operation in this area is completed smoothly and efficiently.