The effect of magnetic field (MF) on the ground state energy of weak coupling polarons in monolayer transition metal dichalcogenides (TMDs) is studied by using LLP unitary transformation and linear combination operator. The dependence of the ground stateenergy of weak coupling polarons on MF, Debye cutoff wave number (DW), internal distance and intrinsic polarizability parameters of the phonon in MF is obtained. The numerical calculation results show that the ground state energy of the weak coupling polarons increases with the increase of magnetic field and the intrinsic polarizability parameter, and decreases with the increase of DW and internal distance of the phonon.