The white light interferometry for the measurement of low-finesse extrinsic Fabry-Perot interferometer(EFPI) is reported. A broadband light is injected into EFPI,and the reflected light is scanned by another high-finesse tunable Fabry-Perot filter,the periodic spectra of EFPI can be obtained.To measure the cavity length,the spectra are fast Fourier transformed,the absolute length of cavity can be calculated by using the periodic spectra. The measuring accuracy is limited by the length of cavity. The longer cavity has higher measuring accuracy. When the length of cavity are 200 μm,400 μm,and 600 μm respectively,the measured results are agree with the actual length of the cavity.