Abstract: The detection of intermediate products in chemical reactions is of great significance for understanding reaction mechanisms. Due to the characteristics of intermediates, such as low concentration, short lifetime, and diverse species, their detection presents a great challenge. Molecular beam mass spectrometry (MBMS) is a technique used for detecting gas-phase reaction intermediates and free radicals. Currently, the two commonly used ion sources in laboratory-based MBMS are the threshold electron ionization (EI) source and the photoionization (PI) source. Due to the oxidation problem of the hot filament, it can only operate at low pressure (about 10⁻³ Pa), which limits the enhancement of its sensitivity. The light source of the photoionization source can be classified into two types: synchrotron radiation sources and low-pressure inert gas discharge vacuum ultraviolet (VUV) lamps. Synchrotron radiation sources offer high energy resolution and tunable photon energy, but their application range is limited due to their high cost. In this work, a novel photoionization molecular beam time-of-flight mass spectrometer (TOF MS) based on a VUV lamp was developed for the efficient detection of active intermediates in chemical reactions. To enhance detection efficiency, a molecular beam photoionization mass spectrometry interface for the reaction chamber was designed. By integrating the reaction chamber with the sample inlet micropore of the mass spectrometer, the transport path of active intermediates was significantly shortened, which was beneficial for efficient extraction and ionization of reactive species. Moreover, the VUV light source was designed with vertical side illumination, which effectively avoided interference from photoelectron ionization, thereby ensuring more accurate measurements. Additionally, the cylindrical electrode focusing design and the increased ionization chamber pressure (0.3 Pa) greatly improved the sensitivity of the mass spectrometer. The standard curve and quantitative information of toluene showed a good linear relationship. The coefficient of determination (R²) was 0.9984, and the limit of detection of toluene was determined to be 0.63 mg/m³. To demonstrate the analytical capability of the instrument for active intermediates, a methyl iodide photolysis experiment and plasma-assisted methane conversion experiment were conducted. The iodine radical (I•) was successfully detected in the photolysis reaction, and various free radicals and active intermediates formed during the plasma-assisted methane conversion reaction were also successfully identified. By detecting free radicals (•CH₃, •OH, O•), active intermediates (CH₃OOH), and products (CO, HCHO, CH₃OH, CO₂, HCOOH), the reaction mechanism of methane-oxygen plasma reactions was clarified, with the gas decomposition mechanism identified as the primary reaction pathway. These experimental results fully demonstrate the great potential of the VUV photoionization molecular beam time-of-flight mass spectrometer for detecting active intermediates in chemical reactions, especially in the study of complex reaction mechanisms, and highlight its significant application value.