The invention discloses an online 
measurement device and measurement method for the NO concentration in a high-NO2 and low-NO 
atmosphere. The online 
measurement device comprises an 
ozone generation unit, a connection 
pipe cleaning unit and a sampling 
pipe, wherein a connection reaction bin, an NO2 
measuring instrument, a second 
mass flow meter and a first 
air pump are sequentially connected behind the sampling 
pipe; and the connection pipe cleaning unit comprises an electromagnetic three-way valve, an 
activated carbon tube, a current-limiting hole and a second 
air pump. By adopting a cavity-
enhanced absorption spectroscopy, a sampling mode (actual 
atmosphere of 
ozone is added) or a zero point mode (actual 
atmosphere) is set through the electromagnetic three-way valve; absorption spectrum intensity signals of two 
modes are measured through an NO2 
measuring instrument as a sampling spectrum and a reference spectrum respectively, and then NO in the atmosphere is converted into a concentration value of the NO2 through 
spectral fitting, that is to say, measurement of the NO concentration in the atmosphere environment is achieved, so that the online 
measurement device and measurement method have the characteristics of being free of NO2 concentration interference, a low 
detection limit, high sensitivity, high temporal-spatial resolution, high stability and low cost.