The invention provides an apparatus for evaluating PM 2.5 degradation effect by a photocatalysis material. The apparatus comprises a gas reaction chamber, a mixing compressed gas generation apparatus, and a gas analyzer; wherein the gas reaction chamber is produced by employing a colorless transparent material. The invention also provides a method for evaluating the degradation effect of harmful gas in atmosphere by the photocatalysis material through the evaluation apparatus, the mixing gas is introduced into the gas reaction chamber, before the gas reaches a stable mode, the gas reaction chamber is covered by a lightproof cover, the concentration of the outlet gas is controlled by controlling the flow of a flowmeter; after the gas concentration is reached, the lightproof cover is removed, the concentration change in the gas reaction chamber is monitored and recorded, the gas is continuously introduced during a monitoring process, and then the illumination intensity is continuously monitored and recorded; after the gas in the reaction chamber is stabilized, monitoring is stopped; and the degradation effect of the harmful gas by the photocatalysis material is calculated. The evaluation apparatus can simulate the real state of the pavement, and the evaluation result is close to the real condition.