In this paper, the application of femtosecond optical frequency comb for length measurement is analyzed and developed. Based on the modified Michelson interferometer by combining with multiple pulse train interference and time-of-flight method, an arbitrary and absolute length measurement system is set up. The timing difference is calculated by the firstorder and second-order optical cross-correlation signals separately; therefore the absolute distance is obtained. An experiment of measuring a 60cm length is conducted and the results are compared with that from a high precision laser displacement sensor. Results show a good measuring linearity with the measurement precision of ±0.5μm.