The invention discloses multifunctional concrete synergist and a preparation method thereof. The multifunctional concrete synergist is mainly prepared by the followings steps: performing the etherification reaction on starch and an alkyl etherifying agent at 30-70 DEG C for 8.0-18.0 hours to obtain a starch-based alkyl etherification product; performing the etherification reaction on the starch-based alkyl etherification product with unsaturated acid and triethanolamine to obtain an esterification product 1; performing the etherification reaction on unsaturated acid and triisopropanolamine to obtain an esterification product 2; performing grafting reaction on the esterification product 1 and the esterification product 2 in a reaction kettle at 50-90 DEG C for 1.0-4.0 h. The density of the multifunctional concrete synergist is 1.005+/-0.020, the pH value of the multifunctional concrete synergist is 10.0+/-1.0, the chlorine ion content of the multifunctional concrete synergist is less than or equal to 0.30%, the sulfate ion content of the multifunctional concrete synergist is less than or equal to 1.00%, and the alkali content of the multifunctional concrete synergist is less than or equal to 1.00%. The multifunctional concrete synergist is mainly used for improving the peaceability of concrete, reducing bleeding and pumping resistance and improving the comprehensive property of concrete.