Abstract: The grouting quality of post-tensioned prestressed ducts is related to the safety of bridges. An inspection system based on coplanar capacitive sensor for grouting quality was presented, which can detect the stratification of grouting in the ducts. With the derived semi-analytical equation for a double-plate capacitive proximity sensor, the optimal parameters of coplanar capacitive sensor were determined. Based on the principle of capacitive proximity sensor, data acquisition unit and data conversion unit were designed. To promote the system, system control unit and data display unit were designed. According to the approximation theory, the system verification test was designed by measuring capacitance of different thicknesses of liquid on the glass surface and comparing the measurement results with the COMSOL simulation results, error analysis and data processing were done. Finally, a system error evaluation was completed. Experiments show that the average relative error of the system is 9.5%, the resolution is 0.1mm and the system can effectively distinguish stratification of grouting in the ducts. Additionally, the inspection system size (except the sensor) is 100mm×80mm×30mm and is equipped with a portable power that is convenient for online inspection. The system can be used in inspection of grouting quality of post-tensioned prestressed ducts and the test result is reliable and accurate, which can satisfy the requirements of engineering application.