Pipeline vibration induced by fluid flow can cause pipeline fatigue damage, which seriously endangers the safety of the operation. This paper aims to clarify the dynamic variation law of pipeline vibration induced by gas–liquid two-phase flow in the elbow and the influence of different gas–liquid ratios on the dynamic evolution of two-phase flow patterns. The volume of fluid (VOF) method is used to capture the flow pattern characteristics to explore the complex information of dynamic flow pattern evolution. The vibration characteristic mechanism corresponding to the evolution of the flow pattern is explored based on the time and frequency domain, and the origin of the exciting force of the elbow is explored according to the momentum balance equation. The simulation results show that the VOF method can well capture the characteristics of slug flow developed by the interphase instability mechanism. After phase space reconstruction, the dynamic evolution mechanism of the flow pattern is complex and the chaotic characteristics of slug flow are strong. The evolution of the flow pattern is related to the increase in wave height. The superficial velocity of the liquid phase is more sensitive to the flow pattern's formation mechanism than the gas phases. The amplitude of the wave strongly depends on the Vsg and Vsl. There is a strong correlation between the main exciting force pulsation and momentum flux pulsation.