Based on the solution of the density matrix of motion equations we gave a theoretical simulation and experimental measurement of reduction of light propagation for a doped Er+ fiber by population oscillation. The effects of the concentration of doped Er ion density and fiber length were given in the paper. The measured data was agree with theoretical simulation well. Maximum delay is 10.75ms. The delay is tunable by changing modulation frequency and laser input power. The result showed the delay based on slow light in fiber can be proposed a novel optical delay line.