Under the effective mass approximation,an explicit formula is obtained concerning the correlations between the GaAs/GaAlAs quantum well widths of finite potential barrier(FPB) structure and those of infinite potential barrier(IPB) structure under the condition that the ground state energies are equal at zero bias states.The results derived from it match well with the results given by others.Then the influences of applied fields on the ground state energies and envelope wave functions are analyzed,verifying the rationality of using the formula at nonzero bias states.