This invention relates to a circulation use method for tail gas of cracked light oil including: a, mixing light oil, liquid ammonia and petrol coke according to a weight ratio and sending them to a cracking oven for reaction, b, the cracked material is deammoniated and washed to be divided into liquid phase and gas phase, c, the gas phase is cleaned and hydrogen is collected from it, d, the collected hydrogen and nitrogen added from outside source enter into a synthesizing tower to be synthesized to liquid ammonia, e, the synthesized liquid ammonia enters into step a in circulation added withsuitable ammonia from outside to make up of the total volume matching to the volume of light oil and petrol coke in step a, which fully uses tail gas generated in the production process of cracking light oil.