Nasal mucus with a rotten smell may be caused by nasal foreign body, fungal sinusitis, maxillary sinusitis and nasal tumor. 1. Nasal foreign body: when the nasal foreign body stays in the nasal cavity for a long time, there will be pus snot and rotten odor. 2. Fungal sinusitis: low resistance of patients or long-term abuse of antibiotics may cause fungal sinusitis, etc. After suffering from fungal sinusitis, symptoms such as nasal congestion and foul-smelling nasal mucus will appear. 3. Maxillary sinusitis: Infection, allergy and other factors cause inflammatory reaction of maxillary sinus, especially maxillary sinusitis caused by odontogenic infection, due to the presence of anaerobic bacterial infection, there is a lot of nasal mucus with foul smell. 4. Nasal tumor: malignant tumor occurs in the nasal cavity, tumor proliferation process, the nasal environment changes, pathogenic microbial infections, the nasal discharge is generally bloody, with putrid odor. When patients have a rotten odor in the nasal mucus, they should go to the hospital in time for examination and diagnosis, and choose the corresponding treatment under the guidance of the doctor.