The invention belongs to the field of cement production, aims to develop a high-performance and relatively-low-cost cement grinding aid, and provides a liquid cement grinding aid and a preparation method thereof. The cement grinding aid comprises the raw materials such as a TNPA.HCl solution, ethylene glycol, polycarboxylic acid and molasses. The preparation method comprises the following steps: weighing a proper amount of TNPA.HCl powder, adding water, and carrying out sufficient stirring to dissolve the TNPA.HCl powder to obtain the TNPA.HCl solution; weighing polycarboxylic acid powder, and adding the weighed polycarboxylic acid powder into the TNPA.HCl solution; adding ethylene glycol according to a certain proportion; finally, adding molasses according to a certain proportion, and carrying out sufficient stirring to obtain an uniform liquid cement grinding aid product. The product has the characteristics of being low in mixing amount, high in efficiency and low in cost; meanwhile, through adoption of the product, the cement yield can be improved, and the consumption of cement clinkers can be reduced.