Abstract: The hardening mechanism was researched by adding CaO to magnesium oxychloride cement slurry，mixing，molding，and curing in a constant temperature and humidity chamber，determining the setting time and initial strength in order to improve and enhance the rapid hardening properties of the magnesium oxychloride cement at low temperature.Results indicated that：the reaction heat of CaO and H2O induced early reaction then shorten the setting time.When the mixing amount of CaO was 4%（mass fraction） of MgO，the initial setting time was reduced from 593 min to 146 min，the final setting time was reduced from 673 min to 374 min with magnesium oxide activity of 62.24%，and the initial setting time was reduced from 570 min to 126 min，the final setting time was reduced from 641 min to 260 min with magnesium oxide activity of 72.01%，while the initial strength of magnesium oxychloride cement at low temperature were both increased.

Key words: magnesium oxychloride cement, setting time, initial strength