Anti erosion design is usually involved in the hydraulic design of hydraulic structures. It is generally stipulated that the anti-erosion design should be carried out in the area with cavitation number σ < 0. 3 or with the flow velocity greater than 15m/ s. The initial cavitation number is different in different parts of hydraulic structures. The value is analyzed not only through theoretical calculation, but also through pressure reduction model tests in which scale effect inevitably exists. The calculation formulas of primary cavitation number at home and abroad were summarized, and the advantages and disadvantages of different formulas and applicable conditions were put forward. The scale effects of primary cavitation were summarized, providing a new thinking method for the determination of incipient cavitation number in model experiment. The criteria of incipient cavitation in different parts of hydraulic structures were summarized, which provides specific and detailed guidance for corresponding design and research.