High-concentration viscous paste has a large conveying resistance during transport in high-pressure long-distance pipelines. When the pump reverses, the paste in the pipeline flows back, which not only causes violent vibration of the pipeline system, but also reduces the conveying efficiency. To study the transient flow characteristics and backflow of the paste in the pipeline during the reverse process, we established a quasi-two-dimensional model for the paste transportation in a high-pressure long-distance straight pipe based on the constitutive equation of viscoplastic fluid and assumed that the paste in the pipeline has an axisymmetric flow and weak compressibility. The finite-difference method was used to solve the model. Using this model, the pressure and velocity distributions along the pipeline under different working conditions were investigated, and the results indicated that the fluctuation amplitude of the pressure is significantly reduced by friction. The pressure fluctuation along the pipeline decreases approximately as an exponential function. The calculation results agreed well with previous analysis results and experimental data, validating the model. This study provides valuable guidance for analyzing the backflow and impulse of high-concentration viscous paste pipeline transportation systems.