The invention provides a method for reducing nitrogen increase of molten steel in a refined-smelting ladle furnace (LF) in a heating process. The center of each of three electrodes is provided with an argon hole, the circumference of the lower edge of a furnace cover is provided with an argon ring pipe provided with argon blowing holes, and the argon holes and the argon ring pipe are respectively connected through a gas inlet hose and an argon pipeline. In the molten steel treatment process of the LF, argon is blown into the furnace respectively through the electrode argon holes and the argon blowing holes in the argon ring pipe. In the heating process of the LF, the speed of a dust removal blower is reduced to 30-70% of the normal speed, and foaming white lime is added to molten steel by 1-3 batches, wherein 1-3kg of foaming white lime is added to every ton of molten steel for each batch. The invention can control the nitrogen increase mass percent of molten steel for one minute on average in the heating process to be lower than 0.00003%, reduce the nitrogen increase of the LF in the heating process, improve the submerged arc effect, relieve the influence of the molten steel nitrogen increase on the quality of the molten steel, and improve the quality of the molten steel treated by the LF.