An absolute distance measurement method is proposed based on coherent detection by a femtosecond optical frequency comb. The absolute distance measurement is realized by the detection of coherence patterns between measurement signals and reference signals. The measurement model of the first-order coherence function is studied. The measurement system based on an unbalanced Michelson interferometer is constructed. The target distance is obtained by the envelope fitting of the first-order coherence function and by the high-precision extraction of peaks on the overlapped pulse positions. The 3 m absolute distance measurement experiment on a long range guide is designed and a real-time comparison with those by the commercial interferometers is conducted. In addition, the ambient factors and system errors are analyzed based on a large number of experimental results. The errors are eliminated and compensated. The results show that in the 500 min long term measurement process, the maximum measurement error is 5.85 μm and the standard deviation is 2.20 μm as for the proposed method with a measurement range of 3 m.