The invention discloses a preparation method of a composite polymeric fiber for high-shading-rate mesh cloth. The method includes: firstly, preparing titanium-zinc composite nano powder from titanium tetrachloride, zinc oxide, ammonium hydroxide and the like; secondly, subjecting to reaction with benzophenone tetracarboxylic acid dianhydride and chitosan derivatives to prepare a nano hydrophilic ultraviolet absorption additive; finally, subjecting to reaction with polyurethane, acrylonitrile and the like to obtain the composite polymeric fiber. In a preparation process, the hydrophilic performance of an ultraviolet absorbent is improved by nano modification, and blending with other raw materials in a subsequent polymeric fiber preparation process can be well realized. The composite polymeric fiber prepared according to the method has the great anti-ultraviolet performance, addition of other small molecular anti-ultraviolet absorbents is avoided when the composite polymeric fiber is used as a main preparation material of the shading mesh cloth, and a finished product is stable in chemical performance, high in shading rate, high in ultraviolet absorption performance and less prone to migration volatilization and has advantages of high tensile strength, aging resistance, corrosion resistance, radiation resistance, light weight and the like.