The invention discloses self-thermal-insulating anti-cracking mortar used for ferromagnetic building formwork walls and a preparing method thereof, and relates to the technical field of building materials. The mortar includes, by weight, 30-55% of a binding material, 5-25% of magnetic ore powder, 0.5-3% of vermiculite powder, 4-20% of hollow glass microbeads, 15-35% of modified quartz sand, 0.1-1.2% of ecological fibers, and 0.2-1.5% of redispersible latex powder, with the sum being 100%. The binding material, the magnetic ore powder, the ecological fibers and the redispersible latex powder are powder materials. The modified quartz sand, the hollow glass microbeads and the vermiculite powder are fine aggregates, and an admixture is water, with the mass of the admixture being 25-90% of themass of the powder materials. The mortar has advantages including high adhesive force, self thermal insulation, good seepage resistance, high toughness, high waterproof and anti-cracking properties, capability of sound absorbing, good workability, weather resistance, fire resistance, corrosion resistance, good construction performance, and the like.