Abstract: In order to study the influence of local bulge on boundary layer transition, the transition SST model was used to simulate the flow around the convex cylinder at high Reynolds numbers. The flow around convex cylinder in subcritical, critical and supercritical conditions was simulated respectively. The flow characteristics of the cylinder with bulge at different Reynolds numbers and the distribution of the mean pressure and friction coefficient were analyzed. The influence of bulge on the flow separation and transition of cylinder surface was analyzed. The differences of pressure and friction coefficient between two sides of cylinder with and without bulge were compared. The results show that when the Reynolds number of the incoming flow is in the critical region, three counter rotating vortices are formed on the convex surface of the cylinder, and then with the increase of θ, flow separation and flow transition occur. For different Reynolds numbers, the convexity on the surface of cylinder can advance the transition position. With the increase of Reynolds number, the lift force increases gradually.