Severe plant virus diseases lead to poor harvests and poor crop quality, and the lack of effective suppressive drugs makes plant disease control a huge challenge. Natural product-based structural simplification is an important strategy for finding novel pesticide candidates. According to our previous research on the antiviral activities of harmine and tetrahydroharmine derivatives, a series of chiral diamine compounds were designed and synthesized by means of structural simplification using diamines in natural products as the core structure in this work, and the antiviral and fungicidal activities were investigated. Most of these compounds displayed higher antiviral activities than those of ribavirin. Compounds and displayed higher antiviral activities than ningnanmycin at 500 μg/mL. The antiviral mechanism research revealed that compounds and could inhibit virus assembly by binding to tobacco mosaic virus (TMV) CP and interfere with the assembly process of TMV CP and RNA transmission electron microscopy and molecular docking. Further fungicidal activity tests showed that these compounds displayed broad-spectrum fungicidal activities. Compounds , , , and with excellent fungicidal activities against f.sp. can be considered as new fungicidal candidates for further research. The current work provides a reference to the development of agricultural active ingredients in crop protection.