In this paper, a method was reported that used a high precision optical spectrum analyzer to measure transmission resonant frequency of air gap F-P etalon and got its free spectrum range by linear fitting. The spacing of F-P etalon was calculated based on this. The measuring precision of this method was analyzed, which showed that the relative error of F P etalon spacing measurement was 2×10 -3 . A novel method of F-P etalon perpendicularity adjusting was introduced as well.