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Summary High-pressure and low-temperature conditions in deepwater gas production and transportation may cause severe hydrate blockage. At present, the research on hydrate migration and deposition primarily focuses on through-diameter pipes, and there are very few related studies on pipes with diameter reduction. In this study, we used the computational fluid dynamics (CFD)-discrete element coupling method. Considering the interaction between hydrate particles, particles and the pipe wall, and particles and fluid, this study establishes a 3D pipe model under different reducing ratios, simulates the migration and deposition law, and reveals the migration and deposition mechanism of hydrate particles at the reduced diameter of pipes. The results revealed that the reduced diameter affected the flow field distribution, led to the emergence of an eddy zone, which strongly hindered migration, and quickly led to the deposition of hydrate particles. Additionally, this study also investigated the effects of gas velocity, reducing ratio, particle diameter, particle generation rate, and flow direction on the deposition efficiency of hydrate particles at the reduced diameters of pipes. Finally, the calculation formula for the eddy zone at the reduced diameter of the pipe was obtained via regression. The equivalent deposition ratio (EDR*) was proposed as a new parameter to characterize the influence of reduced diameter on hydrate particle deposition and provides a new correlation calculation method for it. The results of this study can provide a valuable reference for the efficient control and design of hydrates.