Abstract: Ecological flow is a crucial indicator for maintaining the ecological functions of rivers and lakes and controlling the intensity of water resources development. To systematically advance the assessment and guarantee of ecological flow in rivers and lakes, the main contents of ecological flow assessment and guarantee are summarized from five aspects: selection of control sections, calculation of ecological flow assessment, analysis of ecological flow guarantee rate, determination of ecological flow targets, and measures for ecological flow guarantee. An implementation pathway for ecological flow assessment and guarantee is constructed from six aspects: guiding ideology, basic principles, division of responsibilities, coordinated operation, refined supervision, and dynamic assessment. Taking the Shihe River in Henan Province as a case study, the Q_p method and the Tennant method were used to calculate the ecological flow for existing water conservancy and hydropower projects and key control sections, and the guarantee level was analyzed. The results show that the ecological flow target for the Nanwan Reservoir section is 0.78 m³/s, with a relatively low daily guarantee rate, which can be significantly improved through measures such as installing ecological units; the ecological flow target for the Pingqiao Hub section is 0.83 m³/s, with a daily guarantee rate 90%, basically meeting the ecological flow guarantee target. Furthermore, the implementation and challenges of ecological flow assessment and guarantee for the Shihe River are analyzed in depth, and specific implementation pathways for ecological flow assessment and guarantee in rivers and lakes are proposed, which can provide a reference for the ecological flow assessment and guarantee endeavors in China′s rivers and lakes.