Abstract A new approach is presented in this paper to link the integrated area of the spectral curve with the molten pool temperature, the grain size and molten pool plasma during laser additive manufacturing. A fiber laser was used to deposit the CoCrMo powder on the Q235 steel substrate. The spectra generated during the additive manufacturing process were collected with an optical spectrometer attached to the laser head. According to the thermal radiation theory and the rapid solidification theory, spectral information was applied to calculate the molten pool temperature and the grain size. The relationship between the molten pool temperature and the integrated area of the spectral curve, as well as the relationship between the grain size and the integrated area of the spectral curve was established, respectively. Meanwhile, the electron temperature and electron density of the molten pool plasma were calculated from two different Cr I spectral lines at 403.292 and 434.429 nm wavelength. The results show that the molten pool temperature and the grain size are closely related to the integrated area of the spectral curve. Furthermore, the electron temperature and electron density also increase with the integrated area of the spectral curve.