Spectrally resolved interferometry(SRI)technology is a high precision laser interferometry technology,whose short non-ambiguity range(NAR)increases the precision requirement of pre-measurement in absolute distance measurement.In order to improve NAR of femtosecond laser SRI the factors effecting NAR is studied in measurement system,synthetic NAR method is presented based on excess fraction method to solve this question.A theoretical analysis is implemented and two Fabry-Perot Etalons with different free spectral range are selected to carry out digital simulation experiment.The experiment shows that NAR can be improved using synthetic NAR method and the precision is the same with that of fundamental femtosecond laser SRI.