Abstract: Underwater X-trees are prone to ongoing corrosion under the action of seawater and internal fluids. To address the problem, simulation experiments for the galvanic corrosion and surfacing metal corrosion of 30CrMo steel/625 alloy for the underwater X (Christmas) tree of the Liuhua 11–1 Oilfield were conducted by means of high temperature high pressure reaction kettle; the micromorphology and chemical composition of corrosion product film were analyzed by scanning electron microscope and energy spectrum analyzer, respectively, and analyzed the heterometallic corrosion mechanism of 30CrMo steel/625 alloy. The studies suggested that 30CrMo steel side of the heterometallic contact surface of the underwater X-tree was more likely to suffer a more severe uniform corrosion, and the 30CrMo steel at the contact position and the surfacing weld had more serious groove corrosion. The causes of heterometallic corrosion in the X-tree include galvanic corrosion and crevice corrosion. After surfacing repair, the unevenness of the metallographic structure in the heat-affected zone also exacerbates the crevice corrosion. The research results showed that the underwater X-tree should try to avoid the heterometallic corrosion caused by the contact of different metals. Best practices now suggest that when repairing a underwater X-tree with local corrosion failure, the surfacing material should be properly selected and adopted fully covered surfacing to avoid the risk of heterometallic corrosion.