The determination of chlorideions(Cl -)plays a crucial role in studying chloride nutrition and salt stressprocesses. However，the sample preparation remains a challenge in detecting Cl-，and there is nounified method forplant，soil，fertilizer and water samples. Based on the mechanism of silvernitrate(AgNO 3)titration for Cl-determination，the mechanism and method of using hydrogenperoxide(H 2O₂)for sample pretreatment was investigated. The results showed that H2O₂，in anon-concentrated acid environment，could not oxidize Cl -tochlorinegas(Cl 2). The hydrogenions(H +) produced from the ionization of H2O₂ could combine with Cl-to formhydrogenchloride(HCl)，which volatilized underheating conditions，leading to loss of Cl-. The sample could be pretreated using H2O₂ under heating reflux and condensationconditions，which not only prevented loss of Cl-but also facilitated subsequent determination of titration end point. The H2O₂ pretreatment method was suitable for determination of Cl-inplants，soil and turbid water samples. This method hadthe advantages of simpleoperation，no secondarypollution，high accuracy and precision of measurement results. This study established a theoretical and practical foundation for developing a unified standard testing method for Cl -in different matrix samples.