Spectral intensity and electron density are two key features of plasma during the laser additive manufacturing of ceramics. Pore and crack defects significantly affect the performance of ceramic parts. We built a platform for plasma monitoring and extracted the porosity defect through a thresholding method. The effects of the process parameters on the plasma spectrum intensity and electron density were studied by analyzing the plasma plume and spectrum. The relationship between the plasma characteristics and the forming defects was then obtained. The experimental results show that both the ejection height and the plasma plume area formed by the ionization of the alumina ceramics steam were larger than those of metal materials. The plasma spectral intensity increased with the increase in the laser power and scanning speed, but decreased with the increase in the powder flow rate. The electron density increased with the increase in the laser power, scanning speed, and powder flow rate. During the forming process, the plasma spectral intensity and the electron density highly correlated with the pore and crack defects.