With the advancement of oil and gas exploration and development into deepwater regions, well testing of ultra-deepwater gas wells faces significant challenges posed by tidal effect and wellbore effect. Tidal effect can cause periodic fluctuations in well testing curves, particularly in the pressure buildup section, severely interfering with the accuracy of productivity evaluation and reservoir parameter interpretation. The tidal signals monitored by the seabed pressure gauge are superimposed on the pressure data in the form of attenuation and time shift, especially in reservoirs with high permeability and good connectivity. This interference is more significant and may lead to incorrect interpretation results. In addition, since the pressure gauge is typically located a certain distance above the production zone, the wellbore flow effect further impacts the accuracy of pressure data, resulting in the absolute open flow (AOFP) calculated based on the measured data of the pressure gauge being unable to accurately reflect the true productivity of the production zone. To eliminate the influence of tidal and wellbore flow effects, this study proposes a productivity evaluation method for ultra-deepwater high gas rate wells based on the tidal data removal method and wellbore modeling. This method first uses the tidal signals directly measured by seabed pressure gauges during well testing as a reference to remove the tidal part from the pressure data, thereby obtaining a smooth and oscillation free pressure curve. Subsequently, wellbore modeling software is introduced to establish a wellbore model from the pressure gauge to the top of the production zone. Based on the actual casing and tubing size parameters, the tubing flow section and casing flow section were divided, and the flow pressure was corrected to obtain a more accurate AOFP result that better represents the well production of the perforation section. This study first analyzes the mechanisms of tidal and wellbore effects on the well testing curves of ultra-deepwater gas wells and compares the advantages and disadvantages of different tidal elimination methods. Then, taking an ultra-deepwater high gas rate well as an example, the tidal data removal method and wellbore modeling are applied to process the well testing data, and productivity evaluation and reservoir parameter interpretation are carried out. The results show that this method can effectively eliminate the influence of tidal and wellbore effects, and improve the accuracy of productivity evaluation to 95%, providing technical support for the efficient development of ultra-deepwater gas fields.