The invention discloses a total-reflection-prism-based small-sized spiral PM2.5 concentration detecting device, and belongs to the field of automatic monitoring of environment. The device comprises a laser, a pinhole, a collimating object lens, a rectangular raster, an imaging object lens, an image sensor and a total reflection prism array, wherein a laser beam emitted from the laser passes through the pinhole to form a point light source; the point light source is collimated by the collimating object lens to form parallel light; the parallel light irradiates to the rectangular raster; the rectangular raster is imaged to the surface of the image sensor through the imaging object lens; light beams between the rectangular raster and the image sensor are turned into a spiral shape in an overlooking direction through the plane reflecting prism array; the laser is arranged to be oblique upwards; the total reflection prism array comprises four triangular prisms of which the cross sections are isosceles right triangles; one right-angle side of each isosceles right triangle and the light beams are positioned in the same vertical surface; the other right-angle side of each isosceles right triangle is perpendicular to the light beams. According to the total-reflection-prism-based small-sized spiral PM2.5 concentration detecting device, the instrument cost can be reduced, the calculation amount can be reduced, the monitoring real-time performance can be improved, and meanwhile, the miniaturization design of the detecting device can be realized.