The invention discloses a micro-expanded toughening slurry and a preparation method thereof. The micro-expanded toughening slurry is prepared from, by weight, 100 parts of oil well cement, 8-15 parts of latex, 1-2 parts of fluid loss agent, 1-1.5 parts of drag reduction agent, 1-2 parts of early strength agent, 2-4 parts of expanding agent, 0.2-0.5 part of defoaming agent and 40 parts of water. The micro-expanded toughening slurry has ultra-low filter loss and meanwhile has the advantages of good toughness, good rheological property, unshrinking volume, high early strength and the like. By adopting the micro-expanded toughening slurry, construction is simple, the technical advantage and the environmental protection advantage are remarkable, and the on-site-implemented well-cementing quality is high.