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High precision displacement measuring system is designed by using a capacitive displacement sensor, which carries on non-contact measurement in a certain machine tool spindle's thermal deformation. System's fundamental and data processing methods have been illustrated through the analysis of object's features. The factor of influenced sensor's measurement accuracy and stability is analyzed as well. Then the paper proposes a technical means to utilizing electronic and digital signal filtering technology for enhancing accuracy of sensors. It provides an alternative technological approach for machine tool spindle's high precision on non-contact measurement. After a long period of trial analysis and system calibration, the resolution of the whole measuring system comes up to 30nm, and linearity is superior to 0.6mum. The requirement of withdrawing precision machine tool in processing spindle thermal deformation's properties is satisfied, and the measurement system also provides a meaningful reference to capacitive displacement sensor in a similar application of non-contact high precision measurement.