Abstract: With the increasingly strict requirements for the concentration of NO_x emission from gas-fired units, the operating condition of the monitoring system has changed such that the original monitoring instruments becomes no longer applicable. By analyzing the NO_x monitoring technology and the flue gas pretreatment technology as well as their adaptabilities, a clear understanding of the current situation of the NO_x monitoring system for gas units is obtained. In compliance with HJ 75-2017 and HJ 76-2017 standard, regarding the issues in existing monitoring system such as large measurement range, poor measurement accuracy for low concentration and lack of NO₂ monitoring functionality a series of improvement measures are put forward, e.g., giving priority to chemiluminescence method in NO_x measuring technology, adding NO₂ monitoring devices, using low range and high precision CEMS instrument and pretreatment device with superior performance , implementation of the lightning protection facilities, etc.. Meanwhile the installation of ammonia escape, oxygen and moisture analysis instrument is taken into account in order to meet the control and monitoring requirements. The results are also applicable to regular thermal power units.