According to synchrotron radiation theory, we derive the absorption coefcient, emission coefficient, and the emission intensity from relativistic electrons in the plasma under the different kinds of magnetic fields. Furthermore, we study the maser effect of the synchrotron radiation. The effects of two kinds of magnetic fields are investigated, the first kind is a curved magnetic field, but its strength is homogenous. The other is a dipole magnetic field. We find some special characteristics in the dipole magnetic field. Because of the uncertainty of the energy distribution spectrum of electrons, we choose three typical energy distribution spectra, namely the power law spectrum, Gauss spectrum and thermal spectrum. By calculating the emission intensity, we find the maser effect of synchron-curvature radiation under several kinds of astronomical condition. It is clearly shown that the maser emission of the synchrotron radiation does exist in plasma environment. These results could give some explanations for the atmospheric radiation of planets and might be helpful to solve the problems of the high brightness temperature of some radio sources.