Abstract: Oil and gas pipeline welding robots operate in harsh environments requiring prolonged continuous operation and stringent welding quality. Reliability is foundamental to ensuring their high-performance functionality. To address the comprehensive reliability evaluation of these robotic systems, this study proposes an integrated methodology combining the technique for order preference by similarity to ideal solution (TOPSIS) with a combined weighting approach. First, reliability is analyzed through three criteria: intrinsic reliability, maintainability (with proposed easy-maintenance metrics), and environmental adaptability. Subsequently, integrated weights for evaluation indicators are determined by synthesizing best-worst method (BWM) and criteria importance through intercriteria correlation (CRITIC) approaches based on the principle of minimum information entropy. A TOPSIS-based comprehensive reliability evaluation model is established, and then this method is validated using a flexible inner welding robot for oil and gas pipelines. The results demonstrate that the proposed TOPSIS-combined weighting method can effectively determine the comprehensive reliability levels, and can provide an actionable reference for the design optimization and failure prevention strategies of oil and gas pipeline welding robots.