Capacitive grating sensor can be used to measure the length, as its capacitance changes with the shift difference. There are several measuring systems consisting of capacitive sensor, but all their basic principles are capacitive differential sensing. The structure, principles and characteristics of capacitive sensors are introduced in this paper. Especially, we designed a high-resolution electronic length measuring system based on Capacitive Grating Sensor. And the measurement error of the system and the reason are analysed. For this system, the measuring range is from 0 to 500mm, the resolution is 1um, the indication error is 5um, the measuring speed is 1m/s. Accurate measurement on account of large displacement is achieved with the system.