<p>With the growing application of hydrogen as a clean energy source, the development of accurate, real-time, and multi-dimensional equivalence ratio measurement for hydrogen combustion has become imperative. This is critical for combustion control, optimization of hydrogen engine efficiency, and reduction of nitrogen oxide emissions. Here, we used femtosecond laser-induced plasma spectroscopy to perform online measurements of the equivalence ratio in H₂/air premixed flames. First, calibration experiments were conducted to establish a quantitative relationship between spectral signals of H₆₅₆/O₇₇₇ and equivalence ratios of H₂/air premixed flames. Based on the calibration curve, 1D online equivalence ratio measurements were performed in both laminar and turbulent premixed flames. Furthermore, local equivalence ratio measurements at different heights were carried out to analyze the spatial distribution of the equivalence ratio in the jet flame. These results were compared with the corresponding ANSYS FLUENT simulation data, showing a good agreement in the trend of the equivalence ratio distribution. This study not only provides a new tool for fundamental research in hydrogen combustion but also offers technical support for future numerical simulations and engineering applications related to hydrogen combustion.</p>