The invention relates to a liquid silane coupling agent. The liquid silane coupling agent structurally includes 1-trimethoxysilylpropyl, 3-triethoxysilylpropyl and 5-tripropyloxysilylpropyl polyisocyanate. During preparation, firstly, 10 to 30 part of trimethoxyhydrosilane, 10 to 30 parts of triethoxyhydrosilane and 10 to 30 parts of tripropoxyhydrosilane are mixed, and then 1 to 2 parts of catalysts and 20 to 40 parts of triallyl isocyanuric acid are added for reaction, and the low-boiling point materials are removed after the reaction is completed. The coupling agent is used as an auxiliaryfor adding and using during the preparation of room temperature vulcanized silicone rubber and hot melt adhesive. The liquid silane coupling agent not only has strong adhesion and thermal stability, but also is compatible with most hot-melt adhesive resins.