The chemical reaction of KCl—O2 system was studied under high pressure and high temperature by using the diamond anvil cell and the laser heating technology. The KCl—O2 sample was heated at 37 GPa ［(1 800±200) K］and then the products were measured by Raman technology at ambient temperature. The Raman test results show that the KCl—O2 system undergoes a chemical reaction at high pressure and high temperature, producing a non-conventional compound KCl3 with a trigonal (P-3c1) structure, a small amount of KClO4, solid Cl2 (Cmca), and possibly existing another non-traditional compound KO4. In this experiment, 11 Raman vibration peaks of P-3c1-KCl3 were measured under high pressure. According to the theoretical calculation of the Raman spectrum of P-3c1-KCl3 in the first principle, the 11 Raman vibration peaks were assigned to the vibration modes. The P-3c1-KCl3 gradually weakens on decompression, and decomposes into KCl and Cl2 below 10 GPa, indicating that P-3c1-KCl3 cannot be stored under ambient pressure. It is difficult to detect the Raman peak of KO4for being disturbed by the Raman peak of diamond under high pressure, and the three Raman vibration peaks of KO4 are detected after opening the diamond anvil cell under ambient pressure. Experiments have shown that the moisture-absorbing KO4 black powder can be stored at ambient pressure. The emergence of novel chemical reaction products of KCl3 and KO4 shows that high pressure promotes oxygen and chlorine forming unconventional pair-anions (O—O pair-anions) and polyanions (Cl—Cl—Cl polyanions) with the negative charge of the fraction, indicating that the two elements have unconventional chemical properties under high pressure. Experiments have also shown that there are several unusual chemical reactions under high pressure. The oxidation state of the reactants and products shows that O gains electrons from zero to negative valence, while Cl loses electrons from negative valence to zero or positive valence state, reflecting that O is more electron-friendly than Cl. These novel chemical reactionsprovide a new pathway to synthesize the polyhalide anions compounds that may have exotic properties.