Abstract: The pipeline transporting of oil must produce mixed oil when it transports two or more kinds of oil with different properties.By operating the mixed oil cutting, the choice of the cutting point will directly influence the quality and economic potential of oil.If calculation is made on the theoretical formula of diffusion by Romerg integration method, the change of mixed oil concentration and the percents of the mixed oil volume will be received.It also can provide exact data for choosing the mixed oil cutting point, and provide a base and help for monitoring and blending the contaminated product the mixed oil.Take the Nanshu products oil pipeline as an example, this paper gives a case calculation with this calculation method.