Abstract: Circulating tumor cells (CTCs), as the seeds of tumor metastasis, manage to escape from immune clearance both in microen-vironment and in peripheral blood. CTCs can also facilitate formation of pre-existing environment, which is the crucial step in metasta -sis. Myeloid-derived suppressor cells (MDSC), as a group of immunosuppressive cells derived from bone marrow cells, may play a key role in this process. In this review, the progress in MDSC and its role in the formation, survival, and immune escape of CTCs were exten -sively discussed to explore a new target for tumor immunotherapy.