A cystic axillary mass in a fetus may be a cystic lymphangioma caused by a fetal lymphatic vessel malformation. Fetal lymphatic malformation, also known as cystic lymphoma, is a rare congenital abnormality of the lymphatic system. It is mainly due to isolation of the primitive lymphatic sacs, lymphatic epithelial hyperplasia, and failure of lymphatic drainage due to lymphatic obstruction, leading to cyst formation. The prognosis for fetuses with lymphatic malformations detected in early pregnancy is poor and may cause fetal mental retardation or structural malformations; for fetal lymphatic malformations diagnosed in late pregnancy, the prognosis is good. The prognosis is good for fetal lymphatic malformations diagnosed in late pregnancy because the fetal nervous system and organs are basically developing normally at this time and can wait to be reassessed after delivery. Fetal structural malformations and chromosomal abnormalities should be ruled out in the prenatal period for cystic axillary masses. If there are no other abnormalities, fetal axillary cystic masses can be treated with surgery, laser therapy, or sclerotherapy as appropriate. In conclusion, the detection of fetal axillary cystic mass requires detailed evaluation by a specialist according to the nature and size of the mass, and should not be overly anxious when it occurs during pregnancy, but should be dealt with positively under the guidance of a doctor.