A three-dimensional coupled flow model was built by CFD software. The model serviced the problem of contaminated oil between two viscosity types of down diesels and one type of forward gasoline in batch pipelining. The calculation results of simulation showed that the viscosity difference from the forward to the last oil dramatically affected the contaminated volume aside in the switching process. The change of lateral contaminated section under two different viscosity could be showed. In the earlier contamination, the down diesel will insert in forward oil by bowing forward surface. After 30 seconds, the bowing forward surface became sharp and stretch to the forward in conical surface. In longitudinal contaminated cross-section, the diesel inserted to the forward oil from the lower part of the pipeline by wedge-shaped striker surface. The contaminated volume decreased with the increase of viscosity difference in side, top and bottom area. The contaminated volume is maximum at the bottom boundary area.