In this paper, a comprehensive numerical simulation, which was verified by measurement and design data, was conducted to investigate the issues of thermal deviation in a 1000 MW tower type boiler. Based on this, the cause of thermal deviation was found: a large swirling momentum intensity in the furnace leads to a great deflection of SOFA and then unburned carbon in fly ash gathering in the furnace center is extremely hard to be burn out. At last, a high temperature flame ring occurs at the gas entrance of platen heater zone, resulting in the thermal load deviation of platen heaters. The influences of deflection angle of SOFA nozzles and the main burners in the primary combustion zone were also investigated. The results show that an optimal deflection angle of SOFA nozzles and the main burners in the primary combustion zone can reduce the thermal deviation by decreasing the swirling momentum intensity in the whole area of full furnace. It suggests that modifying the deflection angle of SOFA nozzles to -18 degrees is the best strategy for realizing a minimum thermal deviation case. The results of test on a real boiler adopting this best strategy show that it is effective to reduce thermal deviation. (C) 2019 Elsevier Ltd. All rights reserved.