The invention discloses a composite cement grinding aid and its preparation method. According to a formula, the composite cement grinding aid comprises: a grinding agent, a reinforcing agent, a surfactant, molasses, N, N-bis(2-hydroxyethyl)isopropanolamine and water. The composite cement grinding aid can enhance the fluidity of materials during grinding, effectively reduce overgrinding phenomenon of a mill, improve adhesion of the materials to a grinding body, a lining board, and a bin partition board as well as the self agglomeration phenomenon of the materials, thus increasing the machine-hour yield of the cement grinding mill by l0%-20%, enhancing the cement specific surface area to 20-40m<2>/Kg, reducing the sieve residue, optimizing cement particle gradation, promoting cement clinker hydration, and improving the cement mixture activity. The 3-day compressive strength is increased by 3-5MPa, and the 28-day compressive strength is increased by 4-8MPa. The clinker consumption is reduced by 6-10%. The cement production cost is effectively reduced, the cement security is improved, and the adaptability of cement and concrete admixtures are improved.