A novel laser-assisted Hα spectroscopy is proposed to measure negative ion density in a hydrogen plasma. The laser-induced photodetachment of negative ions leads to a decrease in Hα intensity due to blocking of the mutual neutralization channel associated with generation of H (n=3) atoms. The relationship between the reduced Hα intensity and the negative ion density is investigated experimentally and analytically. It is observed that the reduced Hα intensity follows the trend in the negative ion density as a function of pressure, indicating that this spectroscopy holds promise for determining the negative ion density. In addition, a departure from linearity between the reduced Hα intensity and the negative ion density is also analyzed because it can affect the quantitative determination of the negative ion density in the laser-assisted Hα spectroscopy. The departure is found to be attributed to the change in the mutual neutralization reaction rates depending on plasma conditions.