Abstract: An atmospheric pressure ion-molecular reaction device has been developed.The design and performance of the device match the high sensitivity and specificity of the mass spectrometer(MS).The molecular types of ion-molecule reaction are expanded through the generation of ions and the introduction of neutral reactive molecules under atmospheric pressure.The ion molecule reaction time can be precisely controlled.At the same time, the organic reaction process can be realized online and can be combined with liquid chromatography, which is important for the comprehension of structure and activity judgment of some active reaction intermediates, thus, expands of the functions of the instrument and promotes its application scope, and a more comprehensive analysis for the active reaction intermediates can be provided for organic reaction researchers.