【Background】Giant ragweed is one of main invasive plants in China and has caused great economic losses. The giant ragweed rust (Puccinia xanthi f.sp.ambrosiaetrifida), a phytopathogenic fungus is a potential new agent for biological controlof this invasive plant, but its infection process is not well understood. 【Method】The infection mechanisms of the rust (Puccinia xanthii sp.ambrosiaetrifidae) and [JP2]its effect on the leaf structure of giant ragweed (Ambrosia trifida) were studied by means of light microscopy[JP], scanning electron microscopy (SEM) and transmission electron microscopy (TEM). 【Result】After the rust formed haustoria, the mycelium grew into the intercellular space of the mesophyll cells. The mycelium could infect one mesophyll cell, the cell wall of which was thickened at the contact position. The branches of peripheral ragweed leaf veins were increased by infection, which led the ragweed leaves to moisture decompensation. Further, membrane damage and, organelle structure instability resulted in the cellular endomembrane system falling into disorder and the organelles becoming destabilized. 【Conclusion and significance】The leaf cell structure was destroyed in the process of ragweed rust infecting, which will provide reference for control of the ragweed.