A pre-milling machine for metallographic microscopic samples comprises a pedestal, tracks are arranged at both sides of the inner side of the upper part of the pedestal, the track is internally provided with a sliding plate, a sliding plate power system for driving the sliding plate to move forwards and backwards is arranged below the pedestal, a sliding bar and a sandpaper clamping mechanism are arranged at the bottom of the sliding plate, a portal frame is arranged at the upper part of the pedestal, a clamping mechanism and a water inlet pipe are arranged on the portal frame, a partition plate sliding groove for allowing the sliding bar to move forwards and backwards is arranged on the partition plate, the periphery of the sliding groove protrudes upwards to prevent the cooling water from flowing into the lower space of the pedestal through the sliding groove of the partition plate, a drainage outlet is arranged at the middle position of the bottom of the partition plate, the portal frame has portal frame openings, threads are arranged in the portal frame openings and the adjacent portal frame openings are communicated with each other by virtue of portal frame opening grooves. Scratches on the surfaces of the samples processed by the device are arranged in the same direction, and the surfaces of the samples have uniform roughness; the samples are mechanically clamped to ensure that the samples and the sandpaper are always in a fit state, thereby saving labor and being reliable; the problems of heat generation of the samples and rinsing of sand grains ground off can be solved by cooling water and the structure of the pre-milling machine is simple and feasible.