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The alteration of oilwell cement due to H2S poses a significant threat to wellbore structural integrity in geothermal environments. However, laboratory studies on the cement deterioration process caused by H2S flow along a leaking channel under high-temperature conditions remain scarce. In this study, computed tomography (CT) scanning was utilized to assess the morphological changes and alteration patterns of oilwell cement caused by H2S flow in multiple dimensions. Additionally, scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) were applied to elucidate the microscale mechanisms responsible for the H2S-driven alteration. The results show that: H2S flow along the cement channel results in increased cement matrix porosity and formation of large pores, which are especially evident in regions adjacent to the channel. Chemical etching and secondary crystal growth contribute to the expansion of channel dimension and roughening of the channel wall. Consequently, the permeability of the cement matrix exhibited a marked increase of 45% over a period of 14 days. At the microstructural level, compared to unaltered oilwell cement, which exhibits a homogeneous texture and fine particle composition, exposure to H2S leads to the formation of a heterogeneous and fractured structure within the cement. As a result of sulfidation reactions, a surface layer approximately 1 mm in thickness forms on the cement, which is depleted in calcium and enriched in silicon. The identification of metallic sulfides elucidated the chemical mechanisms responsible for the deterioration of cement properties. In summary, the flow of H2S through the channel within the cement causes significant alteration of the cement structure compared to other alteration modes.