A compressing diaphragm pump having automatic air expelling and pressure abnormal-preventing features for spray use has a hollow tubular air discharge assembly having a plunger body and compressed spring as well as an air passage pierced at the wall of the water exit port and an air discharge orifice pierced of the central top surface of the air discharge assembly for connecting with the plunger body. When air is mixed within the pressurized water, the resilient force of the compressed spring is bigger than the water pressure of the pressurized water. This allows the air mixed in the pressurized water to get into the air passage and pass the plunger opening of the plunger body via through pore, and is dispelled out of the upper hood via the air discharge orifice of the air discharge assembly.