Using Pseudo Wigner-Ville Distribution （PWVD） in time-frequency analysis of geomagnetic Z-component on magnetically quiet and disturbed days, we discover geomagnetic daily variations with period 6 - 24 hour in all stations. The part with period 2 - 6 hour is not only influenced by external field, but also contains regional information, which shows the characteristics of crustal and induction field. Using Smoothed Pseudo Wigner-Ville Distribution （SPWVD） in time-frequency analysis of geomagnetic Z-component before the Wenchuan MsS. 0 earthquake, we found that the geomagnetic stations near the epicenter recorded an abnormal signal with period 4.4 hour before the major earthquake, and the amplitudes of the signal in different stations decreased with epicentral distance increased.