Abstract: The Chinese medicine injections prepared by the natural products containing sesquiterpenoids caused various adverse reactions in clinical use, among which skin allergic reactions are the most common. However, whether the reason of allergic reaction was related to the three isoprene units contained in the sesquiterpenoids is not clear, so the evaluation of drug safety has important guiding significance. The sesquiterpenoids are small molecular substances, and they are not antigens or haptens. They may induce anaphylaxis reactions by acting mast cells directly. Current research confirmed that Mas-related G protein-coupled receptor-X2 (MRGPRX2) which is a 7-transmembrane G protein coupled receptor on mast cells was a key target mediated allergic reactions induced by many small molecular drugs. Unlike IgE-mediated allergic reactions, pseudo-allergic reaction is related to dosage and dosing rate, and occurs in the first exposure to the sensitizer. In this paper, a series of experiments in vitro found that not all sesquiterpenoids caused anaphylactoid reactions. Ginsenoside Re, ginsenoside Rb1 and germacrone were selected as representative of sesquiterpenoids for calcium imaging assay. The data confirmed that only germacrone activated calcium mobilization through MRGPRX2, causing an increase in intracellular calcium ion concentration in mast cells. Furthermore, the release rate of β-hexosaminidase and the release amount of histamine analysis confirmed that germacrone induced mast cells degranulation directly. Knockdown of MRGPRX2 expression by siRNA and competitive binding experiments against ciprofloxacin were used to prove the target of germacrone was MRGPRX2. The results indicated that germacrone could activate mast cells directly to induce anaphylactoid reaction via MRGPRX2, which might be the reason of skin allergic reactions caused by injections containing germacrone.