According to the fundamentals of estimating maximum mixing depths(MMD's) by the assumption of a dry adiabatic lapse rate,two methods of calculating MMD's,that is,a method of solving dual linear equation groups and a method of stepwise approach,have been presented.By using the latter and the everyday meteorological data f rom Meteorological Observing Station of Lanzhou during 1988~1992,the MMD's in Lanzhou Proper were calculated and their variable characteristics with time were analysed.The results were as follows:Annual variation of monthly mean MMD's accorded with a single cycle pattern,that is,the minimum value(716 m) of monthly mean MMD's occurred in December but the maximum value(2 074 m) of them occurred in May.An annual variable tendency of ten-day meanMMD's almost corresponded with that of monthly mean MMD's except two peak values occurring in the last ten days of April and June respectively.In addition,the MMD's in Lanzhou Proper varied with different synoptic systems.The mean MMD was minimum(735 m) before cold-front passage.But it was maximum(946 m) when Lanzhou Proper was controlled by the front of a high pressure system after cold-front passage.Annual variations of monthly mean concentrations of SO₂,CO and NO_X were also single cycle pattern separately.But their phases were almost contrary to that of annual variation curve of monthly mean MMD's,that is,the three air pollutants were all corresponding to the minimum value of monthly mean MMD's in December.The correlation coefficents between the ten-day mean concentrations of SO₂,CO,NO_X and the monthly mean MMD's were-0.774,-0.827 and-0.743,respectively.These showed that their negative correlations were remarkable.In other words,the MMD's were a primary factor affecting air pollution in Lanzhou Proper.