The invention discloses a greenhouse automatic control system and a control method therefor. The control system comprises a main control circuit board, at least one first temperature sensor, at leastone humidity sensor, at least one temperature adjustment device, at least one humidity adjustment device, and a water hydrogen machine. The water hydrogen machine comprises a methanol water reforminghydrogen production device, a hydrogen fuel battery, and an gas pump. The methanol water reforming hydrogen production device is provided with a first high temperature exhaust gas emission pipeline and a second high temperature exhaust gas emission pipeline. The greenhouse automatic control system also comprises a high temperature exhaust gas heating mechanism. The high temperature exhaust gas heating mechanism comprises a high temperature exhaust gas heating pipeline and a second temperature sensor disposed in the greenhouse. The high-temperature exhaust gas heating pipeline is connected to the second high temperature exhaust gas emission pipeline, and use the high temperature exhaust gas output from the methanol water reforming hydrogen production device for heating the greenhouse. The greenhouse automatic control system provided by the invention can improve the convenience of greenhouse electricity consumption, and does not depend on a power grid. The greenhouse automatic control system has characteristics of quiet power generation, energy saving and emission reduction, no pollution to the environment, and promotes the development of the greenhouse system to be intelligent.