The invention relates to a diaphragm type pressure tank. The diaphragm type pressure tank comprises two half tank bodies made of plastic, wherein each half tank body is provided with a butt joint opening and an arc-shaped bottom, the butt joint openings of the two half tank bodies are opposite and are sealed and fixed in a plastic welding mode, through holes are formed in the bottoms of the arc-shaped bottoms, a valve core is arranged at the through hole of one half tank body to form an air hole, a threaded connecting column is arranged at the through hole of the other half tank body to form a water hole, a bowl-shaped inner film is arranged in the pressure tank, a bowl opening of the inner film is turned outwards to form an inner film connecting edge, a pressing ring is arranged between the two half tank bodies, the pressing ring is clamped when the two half tank bodies are welded and fixed, the pressing ring extrudes the inner film connecting edge to form sealed connection, and the inner film divides the interior of the pressure tank into a water chamber and an air chamber which are independent of each other. According to the diaphragm type pressure tank, the half tank bodies are made of plastic materials, so that water erosion is avoided, the inner wall does not need to be protected, the two butt joints are welded, sealed and fixed oppositely in a plastic welding mode, the sealing effect is good, the overall strength is high after sealing, the pressure balance requirement is met, and the service life is long.