Design work using beam–beam simulation has been done since starting the CEPC and Future Circular Collider (FCC) proposals. In recent [Formula: see text] colliders, a collision scheme with a large crossing angle is popular to overcome the hourglass effect. The crab waist scheme is adopted to further boost luminosity performance. Weak–strong simulations were mainly used in the early stage. Strong–strong simulations have been done since last year for the collision with a large crossing angle. Coherent beam–beam instability has been seen in the simulation for FCC. We discuss the instability for the CEPC. a a A general introduction to the CEPC can be found in Ref. 1.