Fabry-Perot etalon is an important device in many classical and modern optical fields. A simple and stable air gap Fabry-Perot etalon is designed to act as core device of spectrally resolved interferometry for absolute distance measurement of femtosecond pulse laser. The Fabry Perot etalon is placed in the place where two beams of Michelson interferometer converge so as to filter out the femtosecond optical frequency comb. The spectral power density of interference signal measured by femtosecond laser absolute distance is accurately monitored, thus the absolute distance is calculated. The parameter design of the Fabry Perot etalon is introduced in detail. The experiment of regulation system of the Fabry Perot etalon is carried out. The principle of spectrally resolved interferometry for absolute distance measurement of femtosecond pulse laser is analyzed briefly. Experimental results show that the measuring distance within ± 1 mm has the accuracy less than ± 5 μm.