Abstract: Indoor air pollution poses significant health risks due to particulate matter (PM) and gaseous pollutants. Air negative ions (NAIs) generated by high-voltage discharge can effectively remove PM, but require precise monitoring to optimize performance and assess environmental impacts. However, the existing monitor methods like ion counters and ion mobility spectrometry lack comprehensive compositional analysis. This study developed an online mass spectrometry platform to monitor ion products from air discharge, addressing challenges in detecting low mass-to-charge ratio (m/z) ions such as O⁻ and O₂⁻. The platform integrated a gas generator, adjustable discharge chamber, high-resolution negative ion time-of-flight mass spectrometry (TOF MS). The key optimizations included buffer gas selection and radiofrequency (RF) quadrupole voltage adjustment. Using helium as the buffer gas enhanced ion transmission efficiency, enabling O⁻ detection and increasing O₂⁻ sensitivity by 7-fold. Optimizing RF peak-to-peak voltage to 270 V mitigated mass discrimination effect. Further experiments revealed humidity-dependent ion dynamics. O₂⁻ intensity decreases with rising relative humidity (10%-90%), while the intensity of O₂⁻(H₂O)_n clusters (n=1-9) initially increases and then declines, with larger clusters peaking at higher humidity. Electrode material (304/316 stainless steel vs. molybdenum) comparison experiment showed that stainless steel generates more abundant ions, including harmful species like O₃⁻ and NO₂⁻, while molybdenum minimizes these byproducts. This platform provides critical insights into NAIs generation mechanisms and environmental interactions. The findings guide electrode material selection and operational parameter optimization for efficient air purification with reduced secondary pollutant emissions, and also can advance NAIs technology for practical applications in indoor air quality management.