Single　safety　factors　are　used　for　most　stability　assessments　of　tailings　slopes　which　can　not　reflect　the　whole　evolutions　of　stabilities　of　tailings　slopes　subjected　to　variations　of　external　condition.　A　method　of　time-varying　analysis　of　tailings　slopes　was　proposed　to　solve　this　problem　and　the　time-history　of　safety　factor　was　used　to　evaluate　the　stabilities　of　tailings　slopes.　A　time-varying　model　was　established　to　analyze　a　tailings　pond.　Conditions　of　rainfall　infiltration,　water　level　fluctuation　and　the　associative　action　of　both　of　them　were　simulated　by　numerical　method　to　study　the　variation　of　safety　factor　with　time.　The　results　show　that　after　6　hours　the　rates　of　decrease　in　safety　factor　under　different　rainfall　intensities　are　similar.　But　within　6　hours　the　greater　the　rainfall　intensity　is,　the　faster　it　decreases　safety　factor.　The　rain　types　will　affect　the　trajectory　of　the　variation　of　the　safety　factor;　and　the　constant　rain　which　is　widely　studied　is　not　the　most　critical　rain　type.　The　variation　of　the　safety　factor　lags　behind　the　water　level　fluctuation.　And　the　hysteretic　effect　is　weakened　by　the　declination　of　rising　rate,　the　increasing　of　descending　rate　and　the　decreasing　of　the　varying　range　of　the　water　level.　It　is　of　important　significance　for　the　designation　and　operation　of　tailings　ponds.