Reasonable allocation is becoming a research focus in the field of water resources. The authors discuss the whole course of reasonable allocation of water resources by expounding the content, foundation, and approaches. The key point of water resources reasonable allocation is to analyze the balance between supply and demand of water resources. In line with social economy and development plan, water resources are allocated from various aspects including quality, quantity, space, and time. We also analyze the target, scheme, and decision-making of water resources reasonable allocation, express related factors as optimized vectors, and establish an allocation model to make a general and reasonable decision. 