  We introduce simple prescriptions of the Yukawa potential to describe the
effect of size polydispersity and macroion shielding effect in charged
colloidal systems. The solid-liquid phase boundaries were presented with the
Lindemann criterion based on molecular dynamics simulations. Compared with the
Robbins-Kremer-Grest simulation results, a deviation of melting line is
observed at small $\lambda$, which means large macroion screening length. This
deviation of phase boundary is qualitatively consistent with the simulation
result of the nonlinear Poisson-Boltzmann equation with full many-body
interactions. It is found that this deviation of the solid-liquid phase
behaviour is sensitive to the screening parameter.
