Long-term chemotherapy and immunosuppressants in acute myeloid leukaemia (AML) patients can result in a high risk of opportunistic infections. Rhizomucor pusillus is an opportunistic pathogen that exists in nature, but infection caused by R. pusillus is rare in the clinic. Notably, the sensitivity and detection time of conventional diagnostic tools for this fungus usually falls short of the needs of clinical diagnosis, resulting in treatment failure. Currently, metagenomics next-generation sequencing (mNGS) has played an important role in the detection of pathogens. Here, we report a case of R. pusillus pneumonia in a haematopoietic stem cell transplantation (HSCT) patient, detected by the mNGS method.