We present a simple method of the spectral interference between two white light continuum spectra to measure the Gouy phase shift of the focused femtosecond laser pulses. From the spectral interference signal, the relative phase is extracted by the Fourier transform method. The minimum spot size (beam waist) of the laser pulses after the lens (focal length f=200 mm) is measured by imaging method. According to the beam radius, the curves of nonlinear fitting are drawn for comparison, which fit the data of experiments very well. The quantity of phase shift is also discussed. The result shows that this method of measuring Gouy phase shifts is available and stable.