The ground state of the magnetopolaron

which is weakly coupled with the bulk longitudinal optical phonons and strongly coupled with the interface optical phonons

in an infinite quantum well within a magnetic fields are studied by using Tokuda' linear-combination operator and the modified Lee-Low-Pines variational method. The law of the change of the effective mass of the magnetopolaron changing with the quantum well's width and the magnetic fields are obtained. Numerical calculations

taking the AgCl/KI quantum well as an example

are performed. The results show that as the following: 1.The effective mass of the magnetopolaron in the quantum well is composed of two parts:the first part is the effective mass of the magnetopolaron induced by the interaction between the electron and the longitudinal optical phonon

the second part is the contribution of the interaction between the electron and the interface optical phonons and the magnetic fields. We have found that

among the four branches of the interface optical phonons

only the interaction between the electron and two branches of the interface optical phonons with frequency of ω

++

and ω

-+

the magnetic fields has the contribution to the effective mass of the magnetopolaron also

but other two branches of the interface optical phonons with frequency of ω

+-

and ω

--

have not any contribution to the effective mass of the magnetopolaron. 2.The total effective mass of the magnetopolaron will decrease drasticlly with increasing the well's width

at last

it will tend to the limiting value of the bulk case slowly. This shows that

when the well's width is smaller

the contribution to the effective mass of the magnetopolaron by the interaction between the electron and the interface optical phonons is important

on the contrary

the contribution to the effective mass of the magnetopolaron by the interaction between the electron and the longitudinal optical phonon increases with increasing the well's width

and becomes the dominant. 3.The effective mass of the magnetopolaron will increases with increasing the magnetic field. It is indi- cated that the magnetic fields will intensify the interaction between the electron and phonons

and will influence on the interaction between the electron and the interface optical phonons only

but has nothing with the interaction between the electron and the longitudinal optical phonon.