The invention discloses an underground water hardening preprocessor. The underground water hardening preprocessor comprises a worktable I, a worktable II, a pump body, a main board surface cover, supporting arms, an upper integral cover and bars, wherein the main board surface cover is connected with the worktable II through the supporting arms; the pump body is fixed to the main board surface cover; a flowing block of the pump body is fixedly connected with the upper integral cover; the lower portion of the upper integral cover is fixedly connected with the bars; a lower integral cover is arranged below the bars; a magnet is arranged at the bottom of the lower integral cover; the lower integral cover is installed in an integral cover fixing seat; and a sliding groove is formed in the bottom of the integral cover fixing seat and is in sliding fit with a sliding rail fixed to the worktable II. When the underground water hardening preprocessor is used, a stainless steel plate to be coiled is subjected to filtering hole punching, needed filtering holes can be punched in the stainless steel plate at a time, the stainless steel plate is subjected to coiling butt welding finally, and work efficiency is greatly improved.