Observations show that the accretion ows in low-luminosity active galactic nuclei (LLAGNs) probably have a two-component structure with an inner hot, optically thin, advection dominated accretion flow (ADAF) and an outer truncated cool, optically thick accretion disk. As shown by Taam et al. (2012), within the framework of the disk evaporation model, the truncation radius as a function of mass accretion rate is strongly affected by including the magnetic field. We define the parameter β as pm = B2=8π = (1 - β)ptot, (where ptot = pgas + pm, pgas is gas pressure and pm is magnetic pressure) to describe the strength of the magnetic field in accretion ows. It is found that an increase of the magnetic field (decreasing the value of β) results in a smaller truncation radius for the accretion disk. We calculate the emergent spectrum of an inner ADAF + an outer truncated accretion disk around a supermassive black hole by considering the effects of the magnetic field on the truncation radius of the accretion disk. By comparing with observations, we found that a weaker magnetic field (corresponding to a bigger value of β) is required to match the observed correlation between L2-10keV/LEdd and the bolometric correction K2-10keV, which is consistent with the physics of the accretion ow with a low mass accretion rate around a black hole.