The invention relates to an atmospheric particulate matter concentration monitoring device, a to-be-detected gas enters from the environment into the monitoring device to form a gas flow passage, the monitoring device includes a light source for production of light beam, the light beam and the gas flow passage cross, a detection region is formed at the crossing position, one side of the light beam is provided with a first photoelectric detector, a first convex lens and a second convex lens which are in coaxial arrangement are arranged between the light beam and the first photoelectric detector, when atmospheric particulate matters pass through the detection region, the light beam produced by the light source produces scattered light, and the scattered light successively passes through the first convex lens and the second convex lens to gather into the first photoelectric detector. One side, near the detection region, of the first convex lens is a convex side, and can effectively reduce agglomeration of the atmospheric particulate matters on the surface, the first convex lens material is glass or resin, and pollution resistance of the first convex lens is much higher than that of a metal concave reflecting mirror.