The invention relates to a method for treating cold-state lead-rich slag with a hot slag side-blowing reduction furnace, and belongs to the technical field of thermometallurgy. The method comprises the following steps: (1) adding 70-90 t of hot-state lead-rich slag, 3-6 t/h of cold-state lead-rich slag, 1.5-2.5 t/h of limestone, 3-4 h/t of iron calcine, 2-3 t/h of coke nut and 0.8-1.6 t/h of crushed coal; (2) after a siphon mouth lead liquid level rises to the lead liquid flowing height in a lead-rich slag adding process, carrying out lead tapping and ingotting; (3) adding 3-10 t of zinc calcine, 2.2-3.5 t of limestone, 3-6 t of iron calcine, 2-3.5 t of coke nut, and 2.5-4 t of crushed coal into each furnace, and controlling the gas flux, oxygen flux, molten pool temperature and in-furnace negative pressure within 3200 to 3600 Nm<3>/h, 950 to 1200 Nm<3>/h, 1100 to 1300 DEG C and -20 to -40 pa respectively; (4) after the lead-rich slag adding process is finished, continuously adding crushed coal at 1000-1250 DEG C for 1-2 hours' smelting, observing the lead content of molten slag with a slag detection bar till the lead content is lower than or equal to 2.2%, and then carrying out 0.5 hour's heating for slag tapping; (5) conducting waste heat recovery and dust collection on flue gas in a smelting process, and then sending the treated flue gas into a desulphuration system. The method is simple in technology and has the advantages that the conventional technology is sufficiently utilized, the defect in stockpiling of the cold-state lead-rich slag is effectively overcome, the recovery rate of valuable metal is improved, and the like.