Penetration rate, bit footage and cost per meter drilling are main technical and economical indexes of exploration engineering. The indexes depend upon bit selected, drilling parameters used and operation technology. The selected bit should be suitable for hardness, abrasiveness (drillability) of rock to be penetrated. The operation technology mainly depends upon qualification and experience of driller. And selection, definition and their rational combination of the drilling parameters are an important factor, which can not be neglected. Concept about critical drilling parameters has been put. The critical parameters are a red line not to pass, otherwise accident will occur. In normal parameters drilling weight on bit and rotary speed should be rationally combined and flow rate of fluid should guarantee cuttings in the bottom of the hole in normal state in order to obtain good technical and economical results. It is suggested to adjust rational combination of weight on bit and rotary speed and control the appearance of critical drilling parameters with wattmeter seriously and quantity of the drilling fluid in the bottle of the hole with flow meter and to combine drilling with experience with drilling with science.