A dynamical convergence criterion for the real-space multislice method for simulating high-resolution electron microscopy images has been derived. This criterion imposes a practical limitation in determining dynamically the number of terms of the propagation operatorP(R)expanded in the subsequent dynamical diffraction calculations concerned with the phase grating selected, the wavefunction and its variation during propagating after choosing the sampling interval Δ and the slice thickness σ. It has been found that, as long as the dynamic criterion is satisfied, much better agreement with the results using a fast Fourier transform multislice method can be obtained.