Traditional chemotherapy against breast cancer usually accompanied with metastasis. Thus, it is significant to enhance anti-tumor and anti-metastasis efficacy simultaneously. The extracellular environment of most tumors is acidic with pH 6.5–6.9, which play an important role in driving promotion of tumor metastasis. Herein, we developed an intratumoral injectable phospholipids-based phase separation gel (PPSG) co-loaded with 5-fluotouracil (5FU) and magnesium oxide (MgO), termed as 5FU + MgO-PPSG. It was prepared by simple mixing, and showed in the appearance of a clear and yellow solution with good fluidity. After intratumoral injection, PPSG system achieved phase transition and sustained released alkaline substance to neutralize acidic microenvironment in tumor site. Both in vitro and in vivo research showed a strong anti-tumor and anti-metastasis efficacy. 4T1-bearing mice after administration with 5FU + MgO-PPSG was achieved significantly prolonged survival time, compared with other groups. In conclusion, the novel 5FU + MgO-PPSG system might have a great potential to improve treatment method against breast metastasis.