Abstract: With the restriction of the use for bromine flame retardants, organophosphorus flame retardants (OPFRs) have been used as excellent substitutes. However, owing to OPFRs are not chemically bound to the materials, they can be easily released from materials to the environment and be exposed to humans by inhalation, ingestion, and dermal adsorption. Furthermore, OPFRs can cause neurotoxicity, carcinogenicity and reproductive toxicity. According to different types of substituents, OPFRs are mainly divided into alkyls, halogenated and aromatics, which have a great variation in physicochemical properties. Depending on the superiority of high resolution, fast acquisition rates, precise mass full-spectrum acquisition of time-of-flight mass spectrometry, it has been widely used to analyze fragmentation mechanism of complex compound. In this paper, the fragmentation mechanism of 17 OPFRs was investigated by using gas chromatography-quadrupole time-of-flight mass spectrometry (GC-QTOF MS) technology at full-scan mode with EI ionization. The experimental results indicated that the ion fragment of these compounds is relatively high and stable at 50 eV of ionization energy. In general, under EI ion source, the alkyls and halogenated OPFRs undergo three γ-H rearrangements, and provide M-R+2H⁺, M-2R+3H⁺ and M-3R+4H⁺ ion fragments. The chlorinated OPFRs are prone to lose CH₂Cl⁺ and Cl^- groups due to specific chemical structures. The aromatic OPFRs are relatively stable due to the conjugation effect, so the molecular ion is the most abundant ion. In addition, the P—O bond is more easily broken to obtain phenol radical ion fragments with higher abundance. For example, o-, m-, and p-trimethyl phosphate are likely to break up P—O bond, but the abundance of fragments are significantly different. Finally, the three types of compounds are often accompanied by water loss during the fragmentation process and rearrangement. By comparing the ion abundance ratio of each compound fragment, the H₄PO₄⁺ (m/z 99) would be optimal for the quantitation of alkyls and halogenated OPFRs using EI. However, the H₄PO₄⁺ ion does not retain any information of the ¹³C or ²H standard labeled compounds. It may lose meaning when uses isotopically labeled internal standard and isotopic dilution mass spectrum quantification methods to reduce matrix effects if this ion was used for quantitation. The method can provide the basis for the structural identification of such compounds.