In offshore riser systems, the slug flow can aggravate the instability of pipeline system during pigging processes, resulting in problems, such as increased pressure fluctuations and the excessive load of the slug catcher over a short period. Bypass pig can disperse the slug due to the highly velocity passing through the bypass hole, thus mitigating the impact of the slug flow on the pigging process. This paper established a two-dimensional CFD model to study the effect of different bypass rates and inlet gas velocities on the downstream slug dispersion process in vertical riser. The results showed that increasing the bypass rate and inlet gas velocity can improve the dissipation effect of the bypass pig to the slug, but it is necessary to comprehensively consider the pigging efficiency and the changes of outlet fluid mass flow and liquid holdup. The research results can contribute to preventing pigging accidents as well as determining the pigging technology parameters during pigging processes in marine pipelines.