A molecular beam experiment on the formation of H + 3 has been carried out by using tunable vacuum ultraviolet (VUV) synchrotron radiation. Reflectron time-of-flight mass spectrometer is applied to detect the signal of H + 3 . The ionization energy (IE) of H 2 and the appearance energy (AE) of H + 3 are determined to be 15.41 eV and 14.61 eV, respectively, by measurements of photoionization efficiency (PIE) curves. Additionally, two most likely H + 3 formation ways, (H 2 )2 → H 3 + + H and (H 2

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... ) 3 → H + 3 +H+ H 2 , are discussed by ab initio calculations at CCSD(T) and MP2 level with aug-cc-pVQZ basis set and compared with the experimental result. The optimized geometries of species involved in the dissociative photoionization channels are also determined at MP2/aug-cc-pVQZ level.