Abstract: Objective Blending hydrogen into existing urban natural gas pipelines is a critical technological pathway for reducing hydrogen transportation costs, accelerating the large-scale application of hydrogen energy, and aiding the achievement of the "dual carbon" goals. However, there are significant differences in the physical properties between hydrogen and natural gas. Hydrogen blending alters the leakage, diffusion, combustion, and explosion characteristics of the mixed gas, introducing new risks to pipeline infrastructure integrity, operational safety, and the adaptability of end-user equipment. Systematic identification and assessment of the whole-chain risks associated with hydrogen blending retrofit are essential to ensuring its safety, feasibility, and widespread adoption. Methods Through literature review and systematic analysis, this study compares the key physical property differences between hydrogen and natural gas, clarifying the direct impact of hydrogen blending on the safety risk profile of the mixed gas. Based on this, a whole-chain risk analysis framework spanning from resource supply, pipeline transmission and distribution, station operation, to end-use is constructed. Key risk points related to pipeline and station facility integrity, operational conditions and process safety, and the end-user side are systematically outlined. Furthermore, risk assessment methods applicable to hydrogen-blending scenarios and comprehensive risk control strategies encompassing "materials-monitoring-management-standards" are summarized. Results Changes in key properties of hydrogen-enriched natural gas, such as density, calorific value, minimum ignition energy, flame speed, and explosion limits, redefine the safety boundaries of urban gas systems. Existing research indicates that at a 20% hydrogen blending ratio, employing low-strength steel pipes, enhancing sealing, and adopting precise gas mixing processes are viable technical directions. However, aging infrastructure, transient operational processes, and end-user appliance adaptability remain primary risk sources. Existing risk assessment methods and computational fluid dynamics simulation techniques can support risk identification and quantification but require updates to data and models tailored to hydrogen blending characteristics. Risk control necessitates building a comprehensive framework covering the development of hydrogen-resistant materials, intelligent monitoring and early warning systems, specialized operating procedures, emergency response plans, and a full lifecycle standard system. Conclusion The retrofit of urban natural gas pipelines for hydrogen blending is technically feasible, yet safety risks permeate the entire chain, requiring systematic assessment and management. Future research and practice should focus on accumulating long-term operational data for pipelines under various hydrogen blending ratios, developing multi-physics coupled risk assessment models, and establishing specialized standard systems and cross-departmental collaborative regulatory mechanisms covering the entire process of design, construction, operation, and maintenance. This will promote the standardized, large-scale, and safe application of hydrogen-enriched natural gas technology.