Bio‐ink has gradually transited from ionic‐crosslinking to photocrosslinking due to photocurable bio‐hydrogel having good formability and biocompatibility. It is very important to understand and quantify the crosslinking process of photocurable hydrogels, otherwise, bioprinting cannot be standardized and scalable. However, there are few studies on hydrogel formation process and its photocrosslinking behavior which cannot be accurately predicted. Herein, the photoinitiated radical polymerized bio‐hydrogels are taken as an example to establish the formation theory. Three typical crosslinking reactions are first distinguished. It is further proposed that not all double‐bonds consumed during crosslinking contributeequally to polymerization. Then the concept of effective double‐bond conversion (EDBC) is elicited. Deriving from EDBC, several important formation indices are defined. According to theory, it is predicted that slow crosslinking can improve the crosslinking degree. Furthermore, based on the slow crosslinking effect, a new strategy of projection‐based 3D printing (PBP) is proposed, which significantly improved printing quality and efficiency. Overall, this work will fill the gap in hydrogel's formation theory, making it possible to accurately quantify the formation process. A systematic formation theory of photocurable bio‐hydrogels is established, and the formation process of hydrogels is clarified. Based on the theory, the photocrosslinking behavior of hydrogels is predicted accurately. Besides, an original projection‐based 3D printing (PBP) strategy is proposed, which significantly improves the printing resolution, printing efficiency, mechanical properties, and biocompatibility.