The paramyxoviruses represent a large family of human and animal pathogens that cause significant health and economic burdens worldwide. However, there are no available drugs against the virus. beta-carboline alkaloids are a family of naturally occurring and synthetic products with outstanding antiviral activities. Here, we examined the antiviral effect of a series of beta-carboline derivatives against several paramyxoviruses, including Newcastle disease virus (NDV), peste des petits ruminants virus (PPRV), and canine distemper virus (CDV). Among these derivatives, 9-butyl-harmol was identified as an effective antiviral agent against these paramyxoviruses. Further, a genome-wide transcriptome analysis in combination with target validation strategies reveals a unique antiviral mechanism of 9-butyl-harmol through the targeting of GSK-3 beta and HSP90 beta. On one hand, NDV infection blocks the Wnt/beta-catenin pathway to suppress the host immune response. 9-butyl-harmol targeting GSK-3 beta dramatically activates the Wnt/beta-catenin pathway, which results in the boosting of a robust immune response. On the other hand, NDV proliferation depends on the activity of HSP90. The L protein, but not the NP protein or the P protein, is proven to be a client protein of HSP90 beta, rather than HSP90 alpha. 9-butyl-harmol targeting HSP90 beta decreases the stability of the NDV L protein. Our findings identify 9-butyl-harmol as a potential antiviral agent, provide mechanistic insights into the antiviral mechanism of 9-butyl-harmol, and illustrate the role of beta-catenin and HSP90 during NDV infection.IMPORTANCE Paramyxoviruses cause devastating impacts on health and the economy worldwide. However, there are no suitable drugs with which to counteract the viruses. We determined that 9-butyl-harmol could serve as a potential antiviral agent against paramyxoviruses. Until now, the antiviral mechanism of beta-carboline derivatives against RNA viruses has rarely been studied. Here, we found that 9-butyl-harmol exerts dual mechanisms of antiviral action, with its antiviral activities being mediated by two targets: GSK-3 beta and HSP90 beta. Correspondingly, the interaction between NDV infection and the Wnt/beta-catenin pathway or HSP90 is demonstrated in this study. Taken together, our findings shed light on the development of antiviral agents against paramyxoviruses, based on the beta-carboline scaffold. These results present mechanistic insights into the polypharmacology of 9-butyl-harmol. Understanding this mechanism also deepens the host-virus interaction and reveals new drug targets for anti-paramyxoviruses.