The recently discovered neutron star transient Swift J0243.6+6124 has been monitored by the Hard X-ray Modulation Telescope. Based on the obtained data, we investigate the broadband spectrum of the source throughout the outburst. We estimate the broadband flux of the source and search for possible cyclotron line in the broadband spectrum. However, no evidence of line-like features is found up to 150 keV. In the absence of any cyclotron line in its energy spectrum, we estimate the magnetic field of the source based on the observed spin evolution of the neutron star by applying two accretion torque models. In both cases, we get consistent results with B similar to 10(13) G, D similar to 6 kpc and peak luminosity of >10(39) erg s(-1), which makes the source the first Galactic ultraluminous X-ray source hosting a neutron star.