To develop an online monitoring and control system for textile dyeing process, an optical fibre sensing system was designed and developed. This paper focused on the measurement of reactive dye concentration in dyebath using the optical fibre sensing system. The optical fibre sensors were proved to be practicable to measure the dye concentration in the dye concentration range during the dyeing process. The measured data was compared with the calculated value, and the linear relation of absorbance to dye concentration was examined. The experiment included single dyes and mixture with double dyes. Dyes concentration measurement accuracy was discussed. The results show that the absorbance is proportional to dye concentration within actual dyeing concentration range. Measured data of dyes concentration well matched the calculated value. The limitations and the approach for improving this optical fibre sensing system were discussed.