Abstract: The main distillation column of a crude unit at a refinery is a strong-coupled multivariate system.The precision of identifiable models has close relationship with performance of control and ecomonic benefit.Considering the safety and efficiency,test should be performed under closed-loop situations.ASYM method has the potential to change multiple manipulated variables at the same time,providing identified models based on frequency domain methods.This paper uses ARMAX model to describe the main distillation column by ASYM method.As efficient measured data reduce the uncertainty in the process of parameter estimation,this paper quantitatively analyzes accuracy improvement brought by new measured data,using output error as performance index.