The invention relates to an additive capable of enhancing the mechanical properties of a magnesite cementing material. The additive is prepared from nano titanium white, polyurethane, sodium alkyl benzene sulfonate and urea-formaldehyde resin, and the mechanical properties of the magnesite cementing material are changed through the action on the magnesite cementing material by nano titanium white, polyurethane, sodium alkyl benzene sulfonate and urea-formaldehyde resin; through clogging and closing the micropores of the magnesite cementing material by nano titanium white, the compactness is improved, and the binding force among molecules is enhanced; through effectively improving the pore structure of the cementing material by polyurethane and urea-formaldehyde resin, the acting force among bodily form molecules is increased, and the elasticity and strength of glass fibers after a product is formed are enhanced, so that the strength and toughness of the product are improved; through reducing the capillary channels of the cementing material by sodium alkyl benzene sulfonate, the compactness of the product is improved, and furthermore, the binding force between inorganic substances and organic additives can be enhanced, so that the water resistance and strength of the product are improved.