Three months after the cracking furnace of 800 kt/a steam cracking unit was built and put into operation, the furnace lining was inspected after the furnace was shut down for coke burning. It was found that the surface layer of some lining refractory bricks above the coke burning mouth of the furnace was cracked and peeled, which was purple red. Based on the analysis of the whole components of the peeled refractory brick, it is found that the abnormal components are added into the refractory brick. The source of the abnormal components is found from the process flow, and the countermeasures are put forward.