The invention relates to a hardness adjusting method and application field of a fiber cloth, which is particularly applied to the manufacturing of a water pump membrane. The hardness adjusting method is realized through the step that hardness adjusting glue is coated on the surface of the fiber cloth, wherein the hardness adjusting glue is made by mixing 90-110 parts of polymer, 90-110 parts of a reinforcing filler agent, 2.5-4 parts of an aging resister, 30-40 parts of a plastifier, 5-7 parts of a novel environmentally-friendly adhesive, 4-5.5 parts of a nanometer active agent and 4-6 parts of a vulcanization system, and the fiber cloth having the adjusted hardness can be used for manufacturing the water pump membrane. According to the hardness adjusting method disclosed by the invention, the flexibility and the hardness of the fiber cloth manufactured by the hardness adjusting method disclosed by the invention are well controlled; the hardness of the manufactured water pump membrane is effectively controlled and is stabilized within a required range; the adhesive force between rubber materials and the fiber cloth is also improved, and the cohesive force is greater than 150N/cm; the problem that the fiber is hardened caused by only coating an isocyanate adhesive is solved; and the control problem of the production technology brought by only adjusting rubber hardness is also further solved.