Natural medicine injections have been widely used in clinics, while adverse reaction reports also have increased rapidly in recent years. To examine the anaphylactoid constituents of natural medicine injections, RBL-2H3 cell degranulation and human serum complement activation models were used to screen the anaphylactoid constituents, and the BN rat model was used to explore the anaphylactoid mechanism of these constituents. The result of an in vitro study showed that the individual compounds of natural medicine injections (chlorogenic acid, hyodeoxycholic acid, cholalic acid, ginkgolic acid, phillyrin, schisandrin B, schisandrin A, puerarin, and tanshinone IIA) and polysaccharide could not induce RBL-2H3 to release histamine and β-hexosaminidase, while proteins Tween-80 and tannic acid were the main anaphylactoid constituents in the natural medicine injections. The in vivo study also indicated that > 10 kDa molecules (proteins) activated classical complement pathways through direct stimulation to cause an anaphylactoid reaction. Tween-80 activated direct stimulation and coagulation pathways through classical and alternative pathways; tannic acid induced anaphylactoid reaction through co-activation of the kallikrein-kinin system, coagulation, integrated, classical and alternative complement pathways. This is the first study to evaluate the anaphylactoid constituents systematically through in vitro and in vivo study. And tannic acid, > 10 kDa molecules (proteins), and injection additives such as Tween-80 are the main anaphylactoid constituents of natural medicine injections.