The 3107 cm-1 peak is observed in the infrared absorption spectra of all types of Ia diamonds, but it has not been observed in the iron-based catalyst. A series of nitrogen and hydrogen-doped diamond crystals is successfully synthesized using P3N5 as the nitrogen source in a catalyst-carbon system at a lower pressure and temperature (6.3 GPa, 1500 ℃). Fourier transform infrared micro-spectroscopy reveals that the hydrogen atoms existing in the synthesized diamond are in two forms. The one is attributed to the CH bond stretching (3107 cm-1) and bending (1405 cm-1) vibrations of the vinylidene group (C＝CH2). The other is due to sp3 hybridization CH bond symmetric (2850 cm-1) and anti-symmetric (2920 cm-1) vibrations. According to our result, we find that the 3107 cm-1 hydrogen absorption peak is related to the aggregated nitrogen in synthetic diamond. The 3107 cm-1 peak could not be observed in synthetic diamond without aggregated nitrogen, even if it has a high nitrogen concentration. And the hydrogen absorption peaks at 2920 and 2850 cm-1 are more widespread than the absorption peak at 3107 cm-1, this suggests that the sp3 CH bond more widely exists in diamond than the vinylidene group (C＝CH2). Infrared spectra analysis indicates that the hydrogen impurity mainly exists in the natural diamond as vinylidene group as seen from the absorption peak intensity. We believe that our results provide a new way to study the formation mechanism of the natural diamond. Moreover, the ideal synthesis condition in our system supplies a possible way for us to design n-type diamond semiconductor.