The invention discloses a H2S monitoring method and system based on downhole while-drilling spectrum drilling process. The method includes the steps: connecting a downhole near-infrared while-drilling spectrum short section with a drill column in a conventional mode, and discharging formation fluids to an annular space from a fluid detection channel outlet after passing a fluid detection channel and a detection module; irradiating the formation fluids flowing in the fluid detection channel by incident near-infrared detection signals, enabling emergent near-infrared detection light signals to a detector, establishing communication among the emergent near-infrared detection light signals and an upper computer through a signal transmission system and finishing while-drilling measurement and analysis; controlling a while-drilling near-infrared measuring tool to simultaneously analyze and display while-drilling testing data by the upper computer according to an electronic device and a data analysis system. By the aid of low-power-consumption, narrow-bandwidth and high-temperature-resistant near-infrared light source device, stable near-infrared light signals with specific wavelength regions are provided and transmit the non-accumulated and zero-lag downhole formation fluids in the fluid detection channel, and an existing ground detection method is changed to finish downhole real-time monitoring.