Abstract: The fourth generation comprehensive model of magnetic field CM4 was used to research the variation of external weak field in magnetically quiet periods over Chinese mainland during 1960-2000. The distribution and variation of ionospheric induced field, toroidal field and magnetospheric induced field separated from the external field were also calculated and analyzed. Results show that in terms of lithospheric induced field, component X's trend of variation was increased firstly, then decreased with time, the amplitude is about 7.3nT; Component Y's trend of variation was initially decreased, and increased with time, the amplitude is about 0.8nT; Component Z's trend of variation was increased and then decreased with time, the amplitude is about 5.5nT. The variation amplitude of toroidal field and magnetospheric induced field of three components are relative small. Their distribution during 1960-1970 and 1970-1980, 1980-1990 and 1990-2000 show an interesting phenomenon that areas of positive and negative values exchanged each other, which may has something related with solar 11 years' activities. Finally, analysis and explaination of these results were given.