In order to solve the problem of secondary lining void and to realize the active monitoring of lining void during casting process, a active monitoring system for lining void prevention based on the distributed compaction sensor is independently developed considering the causes of tunnel lining void. The system consists of distributed compact sensing belt, extension line, direct display terminal and industrial computer with the advantages of simple principle and convenient installation. It is proposed that by embedding distributed compaction sensors among lining layers, active monitoring of lining compactness can be realized during casting. In this way, cavities can be found and grouted not only quickly but also accurately with mould grouting technology, which effectively solve the problem of tunnel lining void. In the validation test, the system identified two vault holes during the casting process, which proved the effectiveness of the system for void monitoring. At present, the system has been applied in some key railway lines in China. The accuracy rate of identification of secondary lining vault void reaches 95.5%.