The utility model relates to an underground steel pipeline cathode protective testing well. The underground steel pipeline cathode protective testing well is characterized by comprising a cathode protective testing pile which is mounted inside the testing well, a well shaft which serves as a well wall, a cast-in-place concrete well base, a well ring which is fixed at the top end of the well base, and a well cover which is flush with the ground, wherein the cathode protective testing pile consists of a testing pile base, a pile cylinder, a PVC wiring pipe, a wiring board and a pile cylinder cover; the well ring is provided with a sealing groove and a threaded through hole; an O-shaped sealing ring is mounted inside the sealing groove; the well cover is fixed on the well ring by adopting a fillister head internal-pentagonal antitheft screw. According to the underground steel pipeline cathode protective testing well disclosed by the utility model, an underground steel pipeline cathode protective testing pile which is generally arranged on the ground is moved to the underground so as not to affect urban landscape or hamper the passing of passengers and vehicles; the underground steel pipeline cathode protective testing well is simple in structure, rapid to install, convenient to use, waterproof, anti-theft and suitable for being used as a cathode proactive effect testing device for an underground steel pipeline which is laid in an urban area and used for oil and gas gathering and transferring, long-distance transportation, urban gas and water supply and urban gas and water discharging.