In an ethylene plant which only takes naphtha as the cracking feedstock, naphtha was gradually replaced by ethane while maintaining the ethylene output unchanged. Through the whole process simulation of separation system, it was found that the loads in quench area, compression zone and hot zone decreased, and bottlenecks first appeared in the passes of hydrogen and circulating ethane in cold box and the stripping section in ethylene rectifier. For compressors, bottlenecks successively appeared in low pressure methane compressor, methane refrigerator and ethylene refrigerator. Replacing no higher than 10% of the naphtha can be achieved by optimizing the operation, and replacing no higher than 20% of the naphtha can be achieved via partial modification..