The invention provides an application of a hollow fiber nano-filtration membrane in removal of trivalent As and pentavalent As in a water body at the same time. The hollow fiber nano-filtration membrane is a low pressure hollow fiber sulfonated polymer coating composite nano-filtration membrane prepared by a coating method, and comprises a hollow fiber ultrafiltration base membrane and an effective separation layer coated on the inner surface of the hollow fiber ultrafiltration base membrane; and the effective separation layer is a sulfonated polymer coating containing manganese dioxide micro-nano particles capable of oxidizing the trivalent As. The hollow fiber nano-filtration membrane can be used for effectively intercepting the As in the water body, and the handling capacity of the As in the water body is 10-10000mu g/L. After the hollow fiber nano-filtration membrane is applied, under the condition that the operation pressure of removing the As in the water body is only 3-10bar, the rejection rate of the As is more than 95%, and the osmosis water yield is high and reaches up to more than 10L/m<2>.h.bar; and compared with a commercial flat-plate nano-filtration membrane, the hollow fiber nano-filtration membrane has the advantages that higher osmosis water flux can be obtained under the lower operation pressure, and meanwhile, the rejection rate of the hollow fiber nano-filtration membrane is equivalent to that of the commercial nano-filtration membrane.