The invention discloses a preparation method of silane paste. The preparation method includes the following steps: 1) preparing materials including: 80.0-85.0 parts of alkyl silane, 0.2-0.4 parts of a compound high-effective self-thickening emulsifier and 14.6-19.8 parts of deionized water, and adding a part of the compound high-effective self-thickening emulsifier to the deionized water, and performing ultrasonic dispersion to obtain a compound high-effective self-thickening emulsifier water solution A; 2) adding the rest of the compound high-effective self-thickening emulsifier to the alkyl silane and performing ultrasonic dispersion to obtain a compound high-effective self-thickening emulsifier alkyl silane solution B; 3) slowly adding dropwise the compound high-effective self-thickening emulsifier alkyl silane solution B to the compound high-effective self-thickening emulsifier water solution A to generate silane paste, and then performing ultrasonic emulsion continuously for 12-18 min, and cooling the silane paste to room temperature and storing the product. Through the ultrasonic emulsion technology, the alkyl silane and deionized water are fully emulsified into a homogeneous high-purity silane paste under effect of trace amount of the emulsifier, wherein the content of the active component, alkyl silane, is more than 80%. The method fully achieves the waterproof and anti-corrosion functions of the alkyl silane.