This paper analyzes the feasibility of reducing the operating pressure in the debutanizer system of an 800 kt/a steam cracking unit which is operated at a high load with heavy feedstock, finds the best operating pressure under the current working conditions, and achieves the goals of slowing down the material polymerization in tower bottom and reboiler, extending the operation cycle of reboiler, optimizing the quality of overhead and bottom products, reducing the energy consumption of the system and improving the economic benefits.