The present invention discloses a silica aerogel modified external thermal insulation rock wool plate preparation method, wherein a modified silica aerogel slurry is prepared, and then the modified silica aerogel slurry is used to treat the rock wool plate. According to the present invention, the thermal conductivity coefficient of the rock wool plate of the present invention is 0.020 W/(M.K) compared with the thermal conductivity coefficient of the existing rock wool plate of 0.040 W/(M.K); the hydrophobic rate can achieve more than or equal to 99%, and the thermal insulation property and the service life of the rock wool plate are improved; and the construction efficiency is high, about 50% of the thickness of the rock wool plate can be reduced through the reduction of the thermal conductivity coefficient, the construction efficiency of workers can be substantially improved, and the comprehensive cost can be reduced.