The relationship between the optical emission spectroscopy and the microstructure of CoCrMoW coatings by laser melting deposition was investigated. The relationship between the primary dendrite spacing and microhardness of CoCrMoW coatings at different laser powers of spectral signal was studied. A new spectral signal index, the integral area, was proposed, and the electron temperature was calculated from four discrete Cr I spectral lines. The results show that with increasing the laser power from 400 W to 1000 W, the average primary dendrite spacing is increased from 3.426 μm to 7.420 μm, and the microhardness HV_0.2 is reduced from 3461 MPa to 3095 MPa. The integral area and electron temperature of the spectral is also increased with increasing the laser power. The primary dendrite spacing of the coating is positively linear-related with the spectral signal, while the microhardness of the coatings is negatively linear-related with the spectral signal. In this research, compared with the electron temperature, the integral area shows a better potential for the prediction of the primary dendrite spacing and microhardness of coatings.