The invention discloses a dust detection method and system and a signal processing unit thereof to detect the dust in the net gas output by different gas-solid separation units. The method according to the invention comprises a signal acquisition section acquiring the target signal with the dust detection components arranged at the net gas output sides of different gas-solid separation units, a signal transmission section transmitting the target signal acquired by the signal acquisition section, and a signal processing section receiving and processing the target signal transmitted by the signal transmission section and outputting the detection results; the dust detection components of the signal acquisition section are arranged on the confluence channel that is connected with the net gas output sides of the different gas-solid separation units, and are respectively arranged behind the connection points on the confluence channel for connecting with the net gas output sides of the different gas-solid separation units.