Abstract: Follicular lymphoma (FL) is one of the frequent varieties of non-Hodgkin's lymphoma (NHL). Secondary skeletal muscle FL is an uncommon extra-nodal lymphoma that is more challenging to identify. Spectral CT imaging can aid in the diagnosis of it. A 52-year-old man who had a bulge in his left inguinal area for two months in 2021. Following a mass aspiration biopsy, cervical lymph node dissection and a bone marrow aspiration biopsy, the patient was diagnosed as follicular lymphoma grade I with bone marrow involvement. Later, the patient had progressive worsening of low back pain for 2 month and underwent a lumbar spectral enhanced-CT scan, which demonstrated the right psoas major muscle and the retroperitoneal swollen lymph nodes were discovered to be homologous lesions, indicating the lymphoma infiltration of the right psoas major muscle. Spectral enhanced-CT is a non-invasive, practical, and reliable alternative technique. By reconstructing and comparing the energy spectrum curves of different tissues to shed light on the nature of the mass, it can aid in the diagnosis of extra-nodal lymphoma, which is of great value in determining the extent of the lesion and guiding the clinical treatment plan.