A cleaning system and a cleaning method are provided. The cleaning system includes a main body, an air suctioning device, a light source, an optical sensor, a memory and a processing unit. The air suctioning device includes an air flow passage and a fan-motor assembly that is disposed in the air flow passage and generates a suction force to suction outside air through the air flow passage. The light source emit light to the air flow passage. The optical sensor captures a plurality of successive image frames from the air flow passage. The processing unit is configured to: obtain first and second image frames from the successive image frames; compare the first image frame with the second image frame to identify dust particles; obtain a particle feature of the dust particles; and determine a current cleanness condition according to the particle feature.