The unknown NO_x distributions inside large-scale CFB (circulating fluidized bed) boilers have always hindered the economy of the SNCR (selective non-catalytic reduction) process. In this study, field tests were carried out on a typical 300 MW CFB boiler, where multi-level 316 L-made probe and Ecom-J2KN/Testo 350 analyzers were used to perform detailed two-dimensional distributions of flue gas composition at SNCR inlets for the first time. The penetration depth inside the horizontal flue pass was up to 7 m. The NO_x distributions were analyzed in detail combining with the auxiliary test in the dilute phase zone. Key results show that the average O₂ concentrations in #A and #C regions were 6.52% and 0.95%, respectively. The vertical NO_x distributions of #A and #C SNCR inlets were similar, showing a trend of first increasing and then decreasing with peak value all appeared at 5 m depth, while the NO_x distribution of #B SNCR inlet was basically increasing. Some local areas with extremely high NO_x concentration (over 2000 mg/m³) were observed near the inclined edge of SNCR inlets, which has never been reported before. Based on this, the optimization of urea injections was conducted, which could save 15.7% of the urea solution consumption while ensuring ultra-low emission of NO_x.