The invention discloses a PM (Particulate Matter) 2.5 particle size cutting device, which can effectively cut particle matters with a particle size of more than 2.5 microns by utilizing the aerodynamics property of the particle matters. The PM 2.5 particle size cutting device comprises an air sample inlet pipe, a cyclone separator, an adaptor and an air sample outlet pipe, wherein the cyclone separator is formed by an air inlet, a separation cavity, an air outlet and a particulate matter collection chamber. The PM 2.5 particle size cutting device is compact in structure and convenient to use, can efficiently cut the particulate matters with the particle size of more than 2.5 microns under the condition of a constant flow rate, and can be suitable for preceding stage particulate size cutting of environment monitoring equipment.