A layered polyimide （PI）-fiber Bragg grating （FBG） concentration sensor was designed to monitor the concentration of a coolant in a nuclear cooling pipeline to solve various problems that exist in current measuring instruments， such as vulnerability to environmental interference， medium deposition， and difficulty in the distribution of multi-point monitoring. First， the design of a layered PI film-FBG concentration sensing mechanism was investigated based on a combination of the PI film water absorption characteristics， the principle of water molecule diffusion， and FBG sensing theory. Subsequently， the layered PI film-FBG concentration sensor was designed and fabricated， the concentration measurement experiment platform was built， and the concentration sensing experiment was conducted. Finally， the concentration characteristic curve of the PI film-coated FBG concentration sensor in a potassium chloride （KCl） solution was obtained， and its sensitivity and hysteresis characteristics were analyzed. The experimental results show that there is a linear relationship between the concentration and wavelength shift， and the fitting degree of the concentration characteristic curve is 0.994 2. In addition， the maximum difference between the forward and reverse stroke output is 35 pm， and the average sensitivity of the sensor is 157.6 pm/（mol·L^-1）， which are 7.4 times and 49.1 times higher than those of the one-layer annular PI film-coated FBG concentration sensor and corrosive FBG concentration sensor under the same conditions， respectively. Therefore， the sensor provides a new method for measuring the coolant concentration in nuclear industrial pipelines with high sensitivity， safety， and anti-electromagnetic interference.