In order to reduce the NOx concentration entering the SCR （ selective catalytic reduction）, a 600 MW supercritical boiler was tentatively retrofitted to be a low nitrogen burner. Primarily, the factors causing high concentration NOx emission were investigated. Accord- ingly three kinds of retrofit schemes were made by a numerical simulation, and eventually, an optimized implementation scheme was select- ed. The detailed retrofit scheme is maintaining the original burner number and position,reducing the primary air flow area （ constant wind speed） and obtaining the main burner region of excess air coefficient being 0.85. The replacement of advanced low NO~ burners are recom- mended, and only 6 separation overfire air outlets （ initial elevation 34. 724 m） are left in the two sides of the wall, and a layer of separated overfire air （12 nozzle） is set at the elevation of 39. 350 m. Over a period of operation after retrofitting,the average concentration of NOx entering SCR is about 400. 7 mg/m^3 ,which is about 12. 8% higher than the numerical simulation results indicating the appropriate signifi- cance of numerical simulation.