The invention relates to a low-
viscosity cyano resin and a preparation method thereof, belonging to the technical field of organic 
high polymer materials. The low-
viscosity cyano resin is prepared by proportionally mixing benzoxazine-ring-containing 
tetra-ortho-
phthalonitrile resin 
monomer and diluter with para-nonyl 
phenol-type 
phthalonitrile monomer. The benzoxazine-ring-containing 
tetra-ortho-
phthalonitrile resin 
monomer and nonyl 
phenol-type phthalonitrile monomer can be blended within a wider proportion range; and the mixture has the advantages of lower and adjustable 
viscosity at low temperature, favorable flowability, low-temperature curing and other 
processing properties. Meanwhile, the addition of the para-nonyl 
phenol-type phthalonitrile monomer has small influence on the 
heat stability of the benzoxazine-ring-containing 
tetra-ortho-phthalonitrile resin. The mixed resin has favorable processability of the benzoxazine resin and excellent temperature tolerance of the cyano resin, has the advantages of low-temperature 
processing, medium-temperature molding and high-temperature application, and can be used in the fields of space navigation and ship structural materials, paints, adhesives, 
electronic packaging materials and the like. The preparation method is easy to control, has the advantages of low 
reaction temperature and energy saving, and is suitable for continuous industrial production.