Abstract: Water is one of the most important compounds and plays an important role in biological process, chemical synthesis and material science. Thus, studies about water and related particles water radical cations are always the top research fields. In this work, water dimer radical cation (H₂O)₂⁺· was prepared using home-made low energy corona discharge device at ambient condition, and its structure was further studied using collision induced dissociation (CID) experiment and isotope labelling method. The dissociation results showed that (H₂O)₂⁺· (m/z 36) will lose OH· and H₂O to give ions of m/z 19 (H₂OH⁺) and m/z 18 (H₂O⁺·), competitively, revealing that (H₂O)₂⁺· can exist in two different structures, including H₂OH⁺—·OH (hydrogen atom bound structure (A)) and H₂O∴OH₂⁺ (oxygen-oxygen atom bound structure (B)). Structure A is asymmetry and structure B is symmetry. Furthermore, isotopic labeled experiments were conducted using deuterated water (D₂O) and ¹⁸O-enriched water (H₂¹⁸O). The dissociation of (D₂O)₂⁺· (m/z 40) gave the corresponding protonated water D₂OD+ (m/z 22) and water radical cation (D₂O)⁺· (m/z 20). Dissociation of (H₂O·D₂O)⁺· (m/z 38) produced the protonated water H₂OD⁺ (m/z 20)/D₂OH⁺ (m/z 21), and water radical cation H₂O⁺· (m/z 18), HOD⁺· (m/z 19), D₂O (m/z 20)⁺·. In ¹⁸O-enriched water (H₂¹⁸O) labelled isotopic experiments, (H₂O·H₂¹⁸O)⁺· (m/z 38) also was dissociated into its corresponding protonated water H₂OH⁺ (m/z 19)/H₂¹⁸OH⁺ (m/z 21) and water radical cation H₂O⁺· (m/z 18) and H₂¹⁸O⁺· (m/z 18). The CID and the isotope labelling results indicated that water dimer radical cation (D₂O)₂⁺·, (H₂O·D₂O)⁺· and (H₂O· H₂¹⁸O)⁺·also exist A and B two structured forms. In conclusion, two structures of water dimer radical cation (H₂O)₂⁺· will further reveal that mass spectrometry may help to elucidate chemical mechanism related with (H₂O)₂⁺· in the biological processes and chemical reactions.