Due to quantum wells new physical properties under the constrained potential limitation on the electron, there is a confined potential along QW growth directions and it must be a strong confined potential. By adding an asymmetrical semi-exponential potential in the QW’s growth direction, asymmetrical semi-exponential quantum well is formed. In order to deepen the understanding of the structural characteristics of asymmetrical semi-exponential quantum well, an anisotropic parabolic potential perpendicular to the growth direction of the quantum well was added, at the same time, the confinement of the magnetic field to the electrons in the quantum well is increased, the influences of magnetic field on the propertiesof the vibrational frequency of the weak-coupling polaron in a GaAs asymmetrical semi-exponential quantum well is studied. The vibrational frequency of the weak-coupling polaron in a GaAs asymmetrical semi-exponential quantum well varying with the two parameters of the asymmetrical semi-exponential confinement potential, the cyclotron frequency of the magnetic field and the confinement strengths of anisotropic parabolic potential in the x and y directions is derived by using linear combination operator and the unitary transformations methods. The numerical calculated results are indicated for a typical GaAs semiconductor asymmetrical semi-exponential quantum well crystal. It is founded that the vibrational frequency of the weak-coupling polaroa in a asymmetrical semi-exponential quantum well is a lifting function of the parameter U0, whereas it is a decaying one of the other parameter σ, when the anisotropic parabolic potential in the x and y directions and the cyclotron frequency of the magnetic field are fixed, the coupling between electron and phonon is strengthened and the formation of polaron is promoted by the change of parameters. The vibrational frequency of the polaron increases with the confinement strengths of anisotropic parabolic potential in the x and y directions when the cyclotron frequency of the magnetic field and two other parameters are fixed. It demonstrates the peculiar quantum size confinement effect. The vibrational frequency of the polaron increases with the cyclotron frequency of the magnetic field when confinement strengths of anisotropic parabolic potential in y directions and two other parameters are fixed, The cyclotron frequency of the magnetic field increases with the increase of the magnetic field, thepolaron will be more constrained when the cyclotron frequency of the magnetic field increases, so will the vibration frequency be strengthened.