The atmospheric corrosion behavior of Mg-alloys AZ31B and AZ91D at alternating low temperatures was investigated via laboratory simulation i.e. a high/low temperature dry-humid alternating test chamber. The corrosion rate of the two alloys was positively correlated with temperature as the results acquired by the mass loss measurement. The morphology analysis showed that the two alloys exhibited obvious localized corrosion in the low temperature environment. Combined with corrosion rate and morphology observation (SEM, CLSM), it was found that AZ31B Mg-alloy was more seriously corroded than AZ91D Mg-alloy at the same temperature, but AZ91D Mg-alloy showed obvious pitting characteristics. The average pitting density and depth of AZ91D Mg-alloy were larger than that of AZ31B Mg-alloy, but the corresponding average pitting volume was smaller. FTIR and EDS analysis showed that Mg(OH)₂, MgO and carbonates were the main components of the corrosion products formed at 15-25^oC, while the content of MgO^{in the corrosion products was higher at lower alternating temperatures (from -5oC to -15°C and -5oC to -25°C). EDS results suggested that Aland Cl were present in the area where AZ91D Mg-alloy pitting was serious. It was considered that the destruction of the oxide scale by Cl- had an important influence on the pitting corrosion of AZ91D Mg-alloy.}