Abstract: Subsea pipeline steels operate under increasingly complex service conditions. In addition to requiring larger diameters, thicker walls, and higher pressure resistance, as well as excellent low-temperature drop-weight toughness, these steels must also possess resistance to sour environments. In this study, X70MS acid-resistant pipeline steel was selected as the research material. Based on the chemical composition of low-carbon acid-resistant steel, an alloy design strategy incorporating Nb addition was adopted. Thermomechanical controlled processing (TMCP) was employed to tailor the microstructure, aiming to achieve an optimal balance between high strength and excellent low-temperature toughness in X70MS. Subsequently, the influence of varying Nb content on the microstructure was investigated. Hydrogen permeation tests and hydrogen-charged slow strain rate tensile (SSRT) tests were conducted to characterize the effect of Nb content on hydrogen embrittlement susceptibility. The results indicate that increasing Nb content refines the microstructure, increases dislocation density, promotes the precipitation of (Nb,Ti)(C,N), and enhances strength. In Solution A, the hydrogen trapping capacity and crack initiation propensity of Nb carbonitrides were lower than those of oxides and sulfides. All experimental steels with varying Nb contents exhibited excellent resistance to hydrogen-induced cracking (HIC), with no hydrogen blistering observed on specimen surfaces. Following immersion testing, hydrogen-induced cracks were consistently found to be associated with inclusions containing oxygen and sulfur. Furthermore, the susceptibility to hydrogen embrittlement increased with rising Nb content. At hydrogen charging times of 5 h and 10 h, the elongation decreased significantly, while the strength was less affected. However, at a charging time of 20 h, both elongation and tensile strength declined sharply due to a significant increase in the content of trapped and free hydrogen.