Abstract: The mass spectrometry behaviors of 19 kinds of pyrethroid pesticides (PyPs) in different ion sources were obtained by analyzing and comparing their ion fragment information in negative chemical ionization (NCI) and electron impact ionization (EI). In this work, the fragment information of precursor and product ions of PyPs in NCI and EI sources was obtained by different instrument acquisition modes. In NCI source, it was easy to lose one hydrogen halide molecule to form a secondary ion when PyPs with cyclopropane structure, such as bifenthrin and cypermethrin. And, the stronger electronegativity of functional group connected to the cyclopropane, the stronger its ionic strength. It was easy to lose one CO₂ to form secondary ions for the others that without cyclopropane structure, such as flucythrinate and fluvalinate. PyPs obtained a hot electron in NCI source, which was unstable after forming an anion and thermal cracking occurs. After cleavage of carbon oxygen bond linked to ester group, a negative pyrethroid alkane radical was formed, and then neutral molecules (such as CO₂, HF, HCl, HCF₃, etc.) were lost to form various fragment ions. In EI source, the ester or benzyl positions of PyPs were prone to cleavage. When the ester group was broken, some PyPs would produce RCO⁺, the common fragment ion of carboxylic acid esters, such as benfluthrin and bifenthrin. However, most PyPs only showed fragment ion R⁺ on mass spectrum, the ion RCO⁺ was not been seen, such as cyfluthrin and flucythrinate. First, the C—O bond of PyPs connected to the carbonyl group underwent α breaking, and then a molecule of CO was lost resulting in a characteristic ion fragment containing a cyclopropane structure. The second was that a hydrogen atom rearrangement of six membered ring occurs after one electron was lost on benzene ring. The m/z 181 and m/z 208 were formed when it contained benzyl phenyl ether and the methyl group had a substituted cyano group, while the m/z 183 was formed when it didn't contain a substituted cyano group.