The invention discloses a liquid grinding aid capable of promoting coagulating, and a preparation method thereof, and belongs to the technical field of cement additive. The liquid grinding aid capableof promoting coagulating is prepared from, by mass, aluminum sulfate of 5% to 15%, magnesium sulfate of 5% to 10%, modified alcohol amine small organic molecules of 40% to 50%, sodium lignosulfonateof 3% to 10%, calcium formate of 1.5% to 3%, and balance deionized water. According to the preparation method, aluminum sulfate and magnesium sulfate are mixed as a coagulating promoting main component, so that cement hydration process is accelerated; chemical imbalance caused by high content aluminium ions is capable of realizing dissolving of gypsum and C3A easily, promoting early stage generation of a large amount of ettringite, and realizing rapid coagulation; the modified alcohol amine small organic molecules are taken as a grinding aid enhancing component, and is high in stability; the preparation technology is simple; reaction is relatively mild; the liquid grinding aid is capable of promoting cement early stage rapid coagulation, and at the same time, early stage strength is ensured, later stage strength increasing is not influenced, and the adaptability on cement is excellent.