The invention relates to a system for simultaneously sampling and detecting flue gas NO and mercury. The method comprises an off-line mercury detection apparatus and a sampling probe gun used for stretching into a flue, a flue gas pipeline of the sampling probe gun is several independently arranged quartz pipelines, an outlet of the quartz pipeline is communicated to a flue gas dedusting, acid-removal and dehumidifying apparatus, a flow control apparatus and a NO detector, the flow control apparatus and the NO detector are connected to a computer; a heating layer used for keeping the sampling probe gun at temperature of more than 120 DEG C is arranged on an inner wall of a housing of the sampling probe gun, an active carbon adsorption pipe is arranged on each quartz pipeline, and the active carbon adsorption pipe is filled by an adsorbent, the adsorbent is divided into the adsorbent which is a mercuric oxide capture layer of KCl and a simple substance mercury adsorption layer of mercury-removal active carbon according to a flue gas passing sequence. The system can simultaneously realize sampling and detection of NO and mercury pollutants at a same sampling point in the flue gas, and can realize flow parallel sampling and constant current sampling, the detection results can be mutually calibrated.