Based on the research platform of position-closed-loop servo control system of servo driving system of numerical control (NC) machine tool, the testing principles of sensor-less of Hall current sensor in servo amplifier, encoder and grating scale were researched, and the feasibility of information of sensor-less used in faults evaluation and relationship between faults and information of sensor-less were explored. Besides, different data sampling methods were presented by aiming at different NC systems. It is the base of performance evaluation of servo driving system of NC machine tool on-line.