The invention discloses a long-carbon-fiber-reinforced nylon composite material with electromagnetic shielding function and a preparation method thereof. The composite material is composed of the following components in percentage by mass: 40-80 wt.% of nylon resin, 20-45 wt.% of long carbon fiber, 2-8 wt.% of alloy powder, 2-10 wt.% of maleic anhydride grafted low-density polyethylene (LDPE-g-MAH) and 0.1-2 wt.% of antioxidant. The preparation method of the composite material comprises the following steps: proportionally and uniformly mixing the nylon, alloy powder, LDPE-g-MAH and antioxidant, carrying came out melt blending on a double screw extruder, and directly extruding the melt into a dipping mold; sending the fiber bundle subjected to surface treatment into the cavity of the dipping mold under the action of tension to sufficiently dip the melt; and pulling the dipped fiber bundle out of the mold, cooling, and granulating to obtain the strip-shaped granules of which the length is 10+/-0.2mm. The composite material disclosed by the invention has the specific function of shielding high/low-frequency electromagnetic waves, and has excellent mechanical properties which are peculiar to long-fiber-reinforced plastics.