The invention relates to an underwater automatic beam grabbing device for a hydraulic structure steel gate. The underwater automatic beam grabbing device aims at solving the problems that an underwater beam grabbing structure is high in hooking and unhooking fault rate, is prone to being clamped in an up-down lifting process to affect the opening and closing operation efficiency of a sluice gate and has certain safety hazards. The underwater automatic beam grabbing device comprises a beam grabbing body matched with the width of the hydraulic structure steel gate. The underwater automatic beam grabbing device is characterized in that a vertical downward guiding rod is arranged at the lower end of the beam grabbing body, and the length of the guiding rod is matched with the depth of a guiding hole in the hydraulic structure steel gate; telescopic mechanisms are respectively arranged at the two ends of the beam grabbing body, guiding wheels are arranged in the telescopic mechanisms, and the guiding wheels are matched with a guiding rail on a gate frame of the hydraulic structure steel gate; transmitting mechanisms are respectively arranged at the two ends of the beam grabbing body, the transmitting mechanisms comprise locking pins, and locking heads of the locking pins are matched with one-way hole grooves in the top surface of the hydraulic structure steel gate; and the locking pins can carry out locking action through a connecting rod mechanism according to position information provided by the guiding rod. Due to the automatic operation, the underwater beam grabbing structure is prevented from being hooked, unhooked and clamped.