Generative AI agents have been widely used in creative activities. Complex creations in the real world require multiple interactions with agents to approach the creative goal gradually. However, there are currently no well-defined frameworks to guide human-agent co-creation in complex creative tasks. In this study, we propose GenFlow, a human-agent co-creation framework, to enhance the generation quality and user experience by providing explainable and controllable mixed-initiative with different granularities at different phases of creation. We demonstrated a GenFlow-based picture book agent and conducted an early user test with 11 participants. The results proved the framework’s effectiveness and usefulness in guiding users’ collaborative and interactive behaviors. Our research emphasizes the importance of explainable and controllable interfaces in human-agent co-creation and proposes design strategies. These strategies contribute to designing AI agents that can effectively support the creation process.