A controlled cooling process of low-carbon alloy steel workpieces and a device thereof are disclosed. The device of the invention comprises an infrared thermometry device, an automatic discharge sorting pipeline, a primary external circulating air fan, a secondary external circulating air fan, a third external circulating air fan, a controlled cooling mesh belt, a suction device, an automatic control device and an air-cooling crawler belt, wherein the front end of the controlled cooling mesh belt is provided with the automatic discharge sorting pipeline, the front end of which is provided with the infrared thermometry device; the primary, secondary and third external circulating air fans are successively arranged along the workpiece transmission direction above the controlled cooling mesh belt; the suction device is disposed between the secondary external circulating air fan and the rear end of the controlled cooling mesh belt; and the rear end of the controlled cooling mesh belt is connected to the air-cooling crawler belt. According to the invention, microstructure homogenization and of low-carbon alloy steel and process stability can be guaranteed, and energy consumption is reduced and production period is shortened.