Concentration of CH4 in ambient atmosphere is quite low (～1.8 ppmv) and mixed well. So observation of CH4 variation with high precision has to rely on high-precision of inversion technique. This paper demonstrates the observation of CH4 variations using high resolution (0.02 cm-1) Fourier transform solar spectrometry, in which a high precision and large scale of retrieval algorithm is investigated. In the CH4VCD (vertical column density) retrieval, both a nonlinear least-square method and an iterative scheme are applied after the measured spectrum is modeled with various a-priori parameters. The XCH4 (column-averaged dry air mixing ratio) is calculated by using the O2 VCD derived from the 7885 cm-1 O2-A band. Fitting errors for both CH4 VCD and XCH4 are less than 1%, and most XCH4 retrievals are within the 4 variations are also investigated based on one typical daily observation. It shows that the daily CH4 VCD decreases with time and the variations of the XCH4 is within 0.02 ppmv.