We propose and experimentally demonstrate a novel approach to measure the Er3+ concentration in Er3+-doped silica fiber by fiber Bragg grating Fabry-Perot (FBG-FP) cavity ring-down spectrum. The relationship between the cavity ring-down time and the Er3+-doped concentration is derived. The results demonstrate that the cavity ring-down time is a function of the temperature of FBG, and an Er3+-doped concentration of 0.3 × 1025 m-3 at the FBG operation temperature of 25℃ is obtained, which is consistent with the commercial Er3+-doped silica fiber parameter. The results obtained have theoretical guidance and develop a new method to measure the ion doped concentration in solid matter.