The invention relates to a controlled cooling technology of medium-thick plate and hot-rolled strip steel production lines in the metal processing field, in particular to a cooler of a medium-thick plate and hot-rolled strip steel online or offline controlled cooling device. The device is arranged at the upper part of a plate strip steel controlled cooling device; and the cooler consists of a water inlet pipe, a U-shaped header, spraying pipes, a spraying pipe mounting plate and an exhaust pipe, and performs high-uniformity and high-strength controlled cooling for the upper surface of plate strip steel. The cooler has the characteristics of small gap and big number of the spraying pipes, good cooling uniformity, high cooling capacity and light equipment weight, and can totally satisfy the demands of a medium-thick plate and hot-rolled strip steel online or offline controlled cooling process.