The invention discloses a grass cutter with an impurity separating function. The grass cutter comprises a grass cutter body, wherein the grass cutter body comprises a charging opening and a discharging opening, the lower end of the discharging opening is connected with a conveying passage, a plurality of spray nozzles are arranged on the internal wall of an entrance of the conveying passage, a shaking screen is arranged below an outlet of the conveying passage, a collecting box is arranged just below the shaking screen, a conveying belt is arranged at the side lower part of the shaking screen,and a screen of the shaking screen is arranged obliquely. According to the grass cutter, the spray nozzles spray water to flush forage containing impurities to the screen of the shaking screen, the impurities fall into the collecting box under the action of the shaking screen, and the forage tumbles onto the conveying belt and enters a next procedure; and the grass cutter is good in separating effect and high in impurity separating effect.