The viscosity of quench oil was high during the early operation of the megaton ethylene plant in Dushanzi. During operation, the designed quench oil viscosity reduction process not only reduced the quench oil viscosity, but also increased the gas phase load of the gasoline fractionator, limiting the operation. Quench oil viscosity reducing tower technology was applied in the expansion in 2019, which effectively solved the long standing problem of high quench oil viscosity and ensured the long-term and stable operation of the ethylene plant. This paper introduces the method for controlling quench oil viscosity, the working principle of viscosity reducing tower and the operation optimization ideas, and analyzes and determines the operation adjustment mode of quench oil system after the viscosity reducing tower is put into operation, so as to solve the problems in the operation of viscosity reducing tower.