Abstract: The fourth generation comprehensive model of magnetic field CM4 was reasonably used in this paper to study the variation of external field in magnetic quiet periods over Chinese mainland during 1960--2000. The distribution and variation of ionospheric field and magnetospheric field separated from the external field in 1960--1970, 1970--1980, 1980--1990 and 1990--2000 were also calculated and analyzed. Results showed that X component of external field variation was increased firstly, then decreased with time. The change was about 32 nT. The variation of corresponding magnetospheric field was increased firstly, then decreased continuously. The change was about 23.7 nT. The Y Component of external field variation was decreased, increased and decreased with time. The change was about 3.8 nT. The variation of corresponding magnetospheric field was generally decreased and then increased with time. The change was about 2.3 nT. The Z Component of external field variation was decreased and then increased with time. The change was about 4.6 nT. The variation of corresponding magnetospheric field was decreased then increased. The change was about 9.3 nT. The intensities of corresponding ionospheric field to the three components were all less than 1 nT during 1960--1970, 1970--1980, 1980--1990 and 1990--2000. They vary with interesting phenomenon between positive and negative values, which maybe have something to do with 11 years' solar activities.