The investigators will conduct randomized and controlled clinical studies in order to preliminarily explore the efficacy and safety of low-dose cyclophosphamide and lenalidomide in maintenance therapy for MM that is not suitable for transplantation in the standard-risk group.

The investigators will conduct randomized and controlled clinical studies to preliminarily explore the efficacy and safety of low-dose cyclophosphamide in maintenance therapy for MM that is not suitable for transplantation in the standard -risk group. Standard risk group MM patients who achieve VGPR or above after initial induction and consolidation therapy will use cyclophosphamide or lenalidomide as maintain therapy for 2 years.

Inclusion Criteria:

1. After induction and consolidation therapy (8 courses of chemotherapy), MM patients in the standard-risk group who were initially unsuitable for transplantation achieved a therapeutic effect of VGPR or above;
2. Secretory MM with measurable indicators;
3. Age ≥ 18 years old, gender unlimited;
4. No obvious dysfunction of heart, lungs, etc. (≤ Grade I);
5. General KPS ≥ 70% (excluding those caused by pathological fractures and bone pain).

Exclusion Criteria:

1. Cytogenetic high-risk patients;
2. Recurrent or refractory MM;
3. Using autologous hematopoietic stem cell transplantation as a consolidation therapy;
4. The therapeutic effect did not reach VGPR or above before enrollment;
5. Asymptomatic MM;
6. No measurable indicators;
7. KPS<50%(excluding those caused by pathological fractures）；
8. Dysfunction of heart, lungs, etc. (> Grade I);
9. Unable to cooperate in observing adverse reactions and therapeutic effects;
10. Pregnancy, breastfeeding, or refusal of contraception by women;
11. There is drug abuse and medical, psychological, or social conditions that may interfere with patients participating in research or evaluating research results;
12. Any unstable or potentially endangering patient safety and compliance with the study.