There is a soft lump deep in the left lumbar subcutaneous area, the possible causes are: fibroplasia, benign tumor, soft tissue sarcoma, etc., you can improve the examination and make a clear diagnosis according to the condition to be disposed of accordingly. 1. Fibroplastic foci: Fibroplastic foci are limited lumps formed by the proliferation of fibrous tissue when the subcutaneous soft tissue is stimulated by certain stimuli, and there are no obvious symptoms when the volume is small, and no special treatment is needed, and regular follow-up can be done. If the size of the lump is large, it can be surgically removed if it produces compression symptoms. 2. Benign soft tissue tumors: including fibroma, lipoma, hemangioma, nerve sheath tumor, etc. The main treatment mode is local excision. 3. Soft tissue sarcoma: soft tissue sarcoma is a malignant tumor originated from mesenchymal tissues, most of them have high degree of malignancy, and after diagnosis, it is necessary to carry out radical wide excision in time, and also need adjuvant treatment such as radiotherapy (e.g. vincristine, mitomycin, etc.) in order to reduce the risk of recurrence. If a soft mass is found deep under the skin on the left side of the waist, it is recommended to go to the hospital as soon as possible, complete the examination, make a clear diagnosis and then dispose of it accordingly in a timely manner.