Abstract: The orbital variations of two balloon satellites, DQ-1A(1990-81B)and DQ-1C(1990-81C), are determined and presented in this paper. Apractical method for determining air density from the decay rate of nearly circular orbit is used and the upper atmospheric densities and variations at heights of 500-900km during the period when solar activity was just at a high level have been obtained from two balloon satellites. The results show that two sets of the density and variations determined respectively from two balloon satellites with different area/mess ratios are good agreement with each other and the variations of density with daily and 27 days periodically time scales of solar activity are very obvious. Acomparison between observed density and the COSPAR International Reference Atmosphere 1972 and 1986 is made in this paper. The analytic results are valuable for the study of space environments and the application of upper atmosphere model.