Abstract: This study aimed to address the issue that valve switching operations at the christmas tree of extra-high-pressure (175 MPa) and ultra-deep (>9 000 m) wells are prone to inducing transient flow pressure fluctuations and to investigate the pressure fluctuation patterns. Based on the principle of similarity, an experimental setup for simulation of transient flow in the christmas tree was designed and constructed, which was capable of simulating field conditions. The variation curves of transient flow pressure inside the christmas tree with time were systematically measured by setting different gas-liquid ratios, flow rates, and valve switching combinations. The experimental results indicated that the gas-liquid ratio was a key factor influencing pressure fluctuations. As the gas-liquid ratio increased, the pressure decline rate accelerated, and the gas-liquid two-phase coexistence state was more likely to induce the water hammer effect. Due to the presence of the liquid phase, the overall system no longer followed a single equation of state, and the pressure exhibited a linear relationship with volume, whereas the pure gas phase strictly complied with the equation of state. Regarding pipe configuration, the transient pressure fluctuations of multi-phase flow were aggravated significantly at the position where the diameter was changed, and variations in the main pipe diameter should be minimized in field design. In terms of flow rate, increasing the flow rate significantly shortened the pressure fluctuation response time and increased the amplitude of steady-state pressure fluctuations. Based on comparative analysis of flow channel combinations, the following conclusions are drawn: For single flow channel operation, the main channel is the first choice, followed by the lower branch, and finally the upper branch; for dual flow channel operation, simultaneous opening of the upper and lower branches on the same side should be avoided, and a reasonable opening sequence helps reduce pressure fluctuations; for three flow channel operation, channels with many turning points or the one closest to the wellhead should not be selected to effectively ensure the overall structural safety of the christmas tree and reduce the risk of structural failure induced by transient flow. The research findings provide an experimental foundation for the design, operation, and maintenance of wellhead equipment under extreme conditions.