Inclusion Criteria:  Subjects must satisfy all the following criteria to be enrolled in the study:  age ≥18 years old, male or female. Subjects with diagnosed relapsed or refractory extramedullary multiple myeloma according to IMWG criteria and have had at least 3 prior lines of therapy Evidence of cell membrane BCMA expression, as determined by a validated immunohistochemistry (IHC) or flow cytometry of tumor tissue(e.g., bone marrow biopsies, or plasmacytoma). Subjects with extramedullary myeloma require extramedullary lesions with a maximum diameter of ≥2cm ECOG score is ≤ 2 Estimated life expectancy ≥ 12 weeks. Subjects should have adequate organ function: Absolute neutrophil count (ANC) ≥1×10^9 /L; absolute lymphocyte count (ALC) ≥0.3×10^9 /L; platelets ≥50×10^9 /L; hemoglobin ≥60 g/L. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 2.5×upper limit of normal (ULN); total serum bilirubin ≤ 1.5×ULN. Creatinine clearance rate (CrCl) calculated according to Cockcroft-Gault formula ≥ 40 ml/min. Fibrinogen ≥ 1.0 g/L; activated partial thromboplastin time (APTT) ≤ 1.5×ULN, prothrombin time (PT) ≤1.5×ULN. SpO2 > 91%. Left ventricular ejection fraction (LVEF) ≥ 50%. 8. The subject and his/her spouse agree to use an effective contraceptive tool or medication (excluding safety period contraception) from the date of the subject's informed consent to one year post CAR T cell infusion.  9. Subject must sign the informed consent form approved by the ethics board in person before starting any screening procedure.  Exclusion Criteria:  The presence of any of the following will exclude a subject from enrollment:  Subjects who are known to be resistant to Selinexor; Subjects who need to use immunosuppressive agents for a long time due to graft-versus-host disease (GVHD) or autoimmune diseases. Subjects have received any anti-cancer treatment as follows: monoclonal antibody for treating multiple myeloma within 21 days before leukapheresis, or cytotoxic therapy or proteasome inhibitors within 14 days before leukapheresis, or immunomodulatory agents within 7 days before leukapheresis, or anti-tumor treatments other than those listed above within 30 days before leukapheresis. Subjects who were receiving a used therapeutic dose of corticosteroid treatment (defined as prednisone or equivalent > 20mg) within 7 days prior to screening, except for physiological alternatives, inhalation, or topical use. Subjects with hypertension that cannot be controlled by medication Subjects with serious heart disease: including but not limited to unstable angina, myocardial infarction (within 6 months prior to screening), congestive heart failure (NYHA classification ≥III), and severe arrhythmias. Subjects with systemic diseases that the investigator determined to be unstable include, but are not limited to, severe liver and kidney or metabolic diseases requiring medical treatment. Subjects with second malignancies in addition to MM within the past 5 years before the screening, exceptions to this criterion: successfully treated cervical carcinoma in situ and non-metastatic basal or squamous cell skin carcinoma, local prostate cancer after radical surgery, and ductal carcinoma in situ of the breast after radical surgery. Subjects with a history of organ transplantation. Subjects have received major surgery within 2 weeks prior to leukapheresis or plan to receive surgery during the study or within 2 weeks after the study treatment (excluding local anesthesia) Subjects participated in another interventional clinical study within 1 month before signing the informed consent (ICF). Subjects with any uncontrolled active infection needed to receive systemic therapy within 7 days before leukapheresis.  Positive for any of the following tests:  Hepatitis B virus (HBV) surface antigen (HBsAg) or hepatitis B core antibody-positive and detectable HBV DNA in peripheral blood Hepatitis C virus (HCV) antibody and hepatitis C virus RNA in peripheral blood Human immunodeficiency virus (HIV) antibody Cytomegalovirus (CMV) DNA Treponema Pallidum antibody Pregnant or lactating women. Subjects with mental illness or consciousness disorder or disease of the central nervous system Other conditions that researchers consider inappropriate for enrollment.