Under the circumstances of magnetic field, high pressure and high temperature, the influence of the electron-spin on the self-trapping energy of the weak-coupling 2-D magnetopolaron was studied by using unitary transformations and the linear combination operator method. Numerical calculation, with InAs semiconductor as an example, illustrated that the self-trapping energy level of the weak-coupling 2-D magnetopolaron was divided into two by the electron-spin of different direction. The split self-trapping energy level was more marked with increasing magnetic field. The self-trapping energy of the magnetopolaron decreased and the ratio of electron-spin energy to the self-trapping energy of the magnetopolaron increased with increasing magnetic field or temperature. The ratio of electron-spin energy to the self-trapping energy of the magnetopolaron was very large, when the magnetic field performed sufficient strong and the temperature performed sufficient high.