The invention discloses a polycarboxylic acid wet-mixing mortar plasticizer and a preparation method thereof. The plasticizer comprises the following components in parts by weight; 100 parts of unsaturated carboxylic acid, 500-850 parts of an unsaturated carboxylic acid derivative, 50-100 parts of unsaturated sulfonate, 3-7 parts of n-caprylic alcohol, 25-35 parts of an initiator, 8000-12000 partsof deionized water and 0-15 parts of vitamin C, and liquid caustic soda is added till a final pH value is 5-7. According to a free polymerization method, a wet-mixing mortar plasticizer product is directly prepared, and the wet-mixing mortar plasticizer product has stable physical states. The wet-mixing mortar plasticizer is of a macromolecule organic structure, is applied to wet-mixing mortar, is not liable to be adsorbed or wrapped by porous substances (such as clay) in aggregate, and thus is good in mortar plasticity maintenance. In addition, the plasticizer is appropriate in reaction condition, free of hot source reaction at a normal temperature, concise in process, environmentally friendly, efficient, and applicable to on-scale industrial production methods.