In order to accurately measure in?situ hydrogen atoms during the combustion process of hydrocarbon fuels, this paper proposes a strategy for transient online detection of hydrogen atoms using a femtosecond?nanosecond laser co?excitation method. In this approach, hydrogen atoms in the ground state are excited to the 2S level through a two?photon process using a 243 nm femtosecond laser. Subsequently, these hydrogen atoms are further excited to the 3P level by a 656 nm nanosecond laser, and the fluorescence emission from the 3P?2S transition is detected, enabling interference?free transient online measurement of hydrogen atoms over a wide equivalence ratio range. Experimental results demonstrate that the femtosecond?nanosecond co?excitation method effectively reduces the interference of hydrogen atoms produced by laser photolysis on the detection of in?situ hydrogen atoms.