The air core is an important phenomenon in a hydrocyclone. The steady state of the air core plays a key role in the performance and separation efficiency of a hydrocyclone. The unsteady structure of the air core reflects the unsteady flow field and exacerbates this unsteadiness, thus affecting the steadiness and homogeneity of the flow field, which will lead to lower separation efficiency. The effects of the ratio of the vortex-finder diameter to the spigot diameter (D-o/D-u) on the steady state of the air core were studied by computational fluid dynamics in this paper. In this approach, the Reynolds stress model used to describe the turbulent fluid flow and the volume-of-fluid multiphase model simulated the interface between the liquid and air core. The results show that the steady state of the air core is mainly decided byD(o)/D-u. Different ranges of D-o/D-u lead to different steady states of the air core. No air core appeared inside the hydrocyclone with a D-o/D-u of 1.2. The discontinuous ones occur when D-o/D-u enlarges to 1.6 and 1.8. Through the continuous air core grown in the hydrocyclone with a D-o/D-u larger than 1.8, the rules of the steady state were different. In addition, the operation conditions of the inlet velocity did not change the trend of the steady state of the air core in a certain hydrocyclone.