The invention discloses a cement grinding aid and a preparation method thereof. The preparation method comprises the following steps: adding alcamine, saturated fatty acid, a dehydrating agent and a catalyst in a molar ratio of 1.0: 1.0-2.4: 1.0-3.0: 0.02-0.05 into a reactor; under N2 protection, controlling the reaction temperature at 120-160 DEG C, performing reduced pressure heating reaction for 6-8h under a stirring condition to obtain a crude product; washing the crude product by using a saturated Na2CO3 solution and distilled water sequentially; evaporating out the dehydrating agent by using a rotary evaporator to obtain a final product. The invention further provides a using method of the cement grinding aid. The using method comprises the following steps: adding the cement grinding aid with a mass percentage of 0.01-0.025% into cement powder, and uniformly mixing. The novel cement grinding aid prepared by performing reduced pressure heating reaction on the saturated fatty acid and the alcamine is beneficial to energy conservation and emission reduction in cement production; raw materials are wide in sources, so that types of the raw materials of the cement grinding aid are widened; the cement grinding aid is applicable to production of the cement powder, and can promote increase in the cement yield, promote increase in the cement strength and reduce the production cost.