The embodiment of the invention discloses a constant negative pressure irrigation system, comprising, an irrigation head, a water storage device and constant negative pressure irrigation equipment, wherein, the water storage device is of a tight closed container, and deposited with irrigation water, the lower portion is provided with a communicating hole used for being communicated with the constant negative pressure irrigation equipment through a communicating pipe, the bottom is communicated with the irrigation head directly or through a connecting pipe. The irrigation head is placed at the location where agriculture crop root grows in soil. The constant negative pressure irrigation equipment may include, a through vessel, a porous medium plate and a U shape container, wherein, the through vessel is in an upside down cup shape, the top is provided with an air outlet mouth, which is connected with the communicating hole in the lower portion of the water storage device, and the bottom is open; the porous medium plate normally adheres to the inside wall of the through vessel lower part, the porous medium plate and inside wall adherence place is tight closed; the through vessel is placed in the U shape container, and in the U shape container, liquid is injected to wet the porous plate. By applying the system, continual stable running of negative pressure irrigation is ensured.