In the present study, we proposed an adjustable apex structure to achieve the goal of online adjustment of the apex diameter of a hydrocyclone. And a computational fluid dynamics (CFD)-based numerical simulation was conducted to study the internal flow field and the separation characteristics of an adjustable apex hydrocyclone. The results show that for the hydrocyclone with the conventional apex, the total pressure drop gradually increases as the diameter of the apex decreases, but it exhibits a general downward tendency after the insertion of the adjustable cone. In addition, the tangential velocity of the adjustable apex hydrocyclone is greater than that of the conventional hydrocyclone in the forced vortex region and smaller in the free vortex region. Moreover, the insertion of the adjustable cone to the conventional apex can increase the total separation efficiency of the hydrocyclone by up to 15.81% without a significant change in the separation sharpness. Display omitted •Performance difference between adjustable apex and conventional apex was compared.•The pressure drop exhibits a decreasing tendency after insertion of the adjustable cone.•The adjustable cone can improve the total separation efficiency.•The influence of insertion depth of the adjustable cone was studied.