The invention provides a shaddock peel extract liquid magnesium oxide retarder and application thereof. The magnesium oxide retarder is obtained by the following steps: washing shaddock peel, choppingthe treated shaddock peel, adding alcohol for extraction, filtering to obtain an extract, heating and concentrating the extract to obtain the retarder. Compared with the existing retarders such as hydroxycarboxylic acid and salts thereof, the magnesium oxide retarder is environmentally friendly, low in price, simple in preparation, safe and non-toxic, has a significant retarding effect, and can significantly prolong the setting time.