Abstract The equilibrium of the Chinese first quasi-axisymmetric stellarator (CFQS) has been investigated by the HINT code. It is found that the stochastization of magnetic field lines expands with the increase in the volume-averaged beta value 〈β〉 in the plasma boundary. In the high-β regime, the generation of large magnetic islands at rational surfaces not only leads to an effective shrinkage of the plasma confinement region but also increases the outward shift of the magnetic axis. With bootstrap current effects, the low-order islands spread over the whole plasma area, leading to a stochastization of magnetic field lines due to island chain overlapping. However, for a flat pressure profile, the magnetic islands are significantly suppressed so that the magnetic surfaces can be fairly maintained. Moreover, the magnetohydrodynamic (MHD) instabilities in the optimized CFQS configurations have been simulated by the MEGA code. The linear properties of unstable resistive MHD modes are studied. The results show that in the CFQS the main MHD behaviour is dominated by the resistive ballooning modes with strong mode coupling. The mode structure and resistivity scaling are consistent with related theories.