The invention relates to a highly stable aqueous chain alkyl silane emulsion and a preparation method. The emulsion includes continuous aqueous phase, dispersed hydrolyzable alkoxy silane oil phase and emulsifier; the weight ratio of hydrolyzable alkoxy silane, emulsifier and water is 10 to 65:2 to 15:20 to 88. The hydrolyzable alkoxy silane oil phase is composed of chain alkyl trialkoxysilane R1Si (OR2) 3, trialkoxysilane R3Si (OR2) 3 or dialkoxysilane R4R5Si (OR2) 2 and the corresponding hydrolysis polycondensation oligomer, wherein, substituents R1 and R4 are C1 to 18 chain alkyls, R2 is methyl, ethyl, propyl, etc., and R3 and R5 are acryloyl oxy propyl, ethenyl, amino propyl, Gamma-glycidyl ether oxyl, etc. The emulsifier is nonionic and ionic complex, including OP series, Span series, Tween series and a composite system composed of sodium dodecyl sulfonate, sodium dodecyl sulfate and dodecyl benzene sulfonate.