Climate　change　and　human　activities　have　been　heavily　affecting　oceanic　and　inland　waters,　and　it　is　critical　to　have　a　comprehensive　understanding　of　the　aquatic　optical　properties　of　lakes.　Since　many　key　watercolor　parameters　of　Qinghai　Lake　are　not　yet　available,　this　paper　aims　to　study　the　spatial　and　temporal　variations　of　the　water　clarity　(i.e.,　Secchi-disk　depth,　Z(SD))　and　suspended　particulate　matter　concentration　(CSPM)　in　Qinghai　Lake　from　2001　to　2020　using　MODIS　images.　First,　the　four　atmospheric　correction　models,　including　the　NIR-SWIR,　MUMM,　POLYMER,　and　C2RCC　were　tested.　The　NIR-SWIR　with　decent　accuracy　in　all　bands　was　chosen　for　the　experiment.　Then,　four　existing　models　for　Z(SD)　and　six　models　for　C-SPM　were　evaluated.　Two　semi-analytical　models　proposed　by　Lee　(2015)　and　Jiang　(2021)　were　selected　for　Z(SD)　(R-2　=　0.74)　and　C-SPM　(R-2　=　0.73),　respectively.　Finally,　the　distribution　and　variation　of　the　Z(SD)　and　C-SPM　were　derived　over　the　past　20　years.　Overall,　the　water　of　Qinghai　Lake　is　quite　clear:　the　monthly　mean　Z(SD)　is　5.34　+/-　1.33　m,　and　C-SPM　is　2.05　+/-　1.22　mg/L.　Further　analytical　results　reveal　that　the　Z(SD)　and　C-SPM　are　highly　correlated,　and　the　relationship　can　be　formulated　with　Z(SD)=8.072e(-0.212)C(SPM)　(R-2　=　0.65).　Moreover,　turbid　water　mainly　exists　along　the　edge　of　Qinghai　Lake,　especially　on　the　northwestern　and　northeastern　shores.　The　variation　in　the　lakeshore　exhibits　some　irregularity,　while　the　main　area　of　the　lake　experiences　mild　water　quality　deterioration.　Statistically,　81.67%　of　the　total　area　is　dominated　by　constantly　increased　C-SPM,　and　the　area　with　decreased　C-SPM　occupies　4.56%.　There　has　been　distinct　seasonal　water　quality　deterioration　in　the　non-frozen　period　(from　May　to　October).　The　water　quality　broadly　deteriorated　from　2001　to　2008.　The　year　2008　witnessed　a　sudden　distinct　improvement,　and　after　that,　the　water　quality　experienced　an　extremely　inconspicuous　degradation.　This　study　can　fill　the　gap　regarding　the　long-time　monitoring　of　water　clarity　and　total　suspended　matter　in　Qinghai　Lake　and　is　expected　to　provide　a　scientific　reference　for　the　protection　and　management　of　the　lake.