To study how to prevent the boiler performance degradation under large primary air ratio (PAR) condition, a three dimensional computational fluid dynamics model was established on the basis of a 660 MW wall-fired brown coal boiler. Accuracy of simulation models was established by carrying out a mesh independence test and a comparison with real-life data. Then it was used to investigate the effects of proposed measures on boiler performance. The distribution profiles of combustion temperature, heat flux, CO mass fraction and unburnt char particles were selected to analyze the boiler performance under different cases. Results show that boiler performance deteriorates evidently when PAR is increased to 0.425, and the overall heat flux decreases by 37.0 MW. The three proposed measures can indeed improve the boiler performance that has been deteriorated when PAR is increased, and the overall heat flux was increased by 6.42 MW, 6.59 MW and 15.53 MW respectively. Among the proposed measures, the method of closing part of secondary air nozzles is recommended in regular boiler operation, due to its operability and convenience. The findings of this research and recommended measures can be used as guidelines in the actual operation in brown coal power plant. (C) 2018 Elsevier Ltd. All rights reserved.