The invention discloses a method for preparing silicone sealant. The method comprises the following steps: adding alpha,omega-dyhydroxy polydimethylsiloxane, filler and dimethyl silicon oil into a vacuum kneading machine, vacuuming, heating and mixing for 2-5 hours; stopping heating, introducing nitrogen to change the vacuum state, adding a silane coupling agent, mixing for 0.5-1 hour at the vacuum degree of -0.06 to -0.1Mpa, introducing nitrogen to change the vacuum state; sealing and cooling to room temperature, subsequently transferring into a stirring machine, adding a cross-linking agent, a thickening agent and a catalyst, vacuuming, and uniformly stirring and mixing so as to obtain the silicone sealant. According to a product of the silicone sealant, the addition amount of the filler is increased, the product stability and the surface smoothness are improved, the thixotropy and the deep curing are also improved, the curing homogeneity is good, the tensile strength, the elasticity and the cohesiveness are greatly improved, expensive white carbon black can be replaced, the filling amount is large, the curing time is short, the construction efficiency is improved, the process is simple and feasible, the production cost is low, and good economic benefits and social benefits are achieved.