Osteosarcoma is an aggressive malignant neoplasm, and it is of great significance to the fabrication and investigation of the anti-tumor mechanism of nanomedicine in the treatment of osteosarcoma. Herein, a cinnamaldehyde polymeric prodrug micelle with pH-sensitive charge-conversion ability (mPEG-b-P(C7-co-CA)) was fabricated, and the anti-osteosarcoma mechanism of mPEG-b-P(C7-co-CA) micelle was investigated. mPEG-b-P(C7-co-CA) micelles were prepared by self-assembly method, and their diameter was 227 nm. mPEG-b-P(C7-co-CA) micelles could regulate the cell cycle and inhibit the proliferation of 143B cells, which was demonstrated by flow cytometry analysis, CCK-8 assay and 5-Ethynyl-2′-deoxyuridine (EdU) staining. The wound-healing assay and transwell assay showed that mPEG-b-P(C7-co-CA) micelles effectively inhibited the migration and invasion of 143B cells. It was proven that mPEG-b-P(C7-co-CA) micelles downregulated the levels of proliferation and apoptosis-related proteins and affected osteosarcoma migration and invasion by inhibiting the epithelial-mesenchymal transition (EMT). In addition, mPEG-b-P(C7-co-CA) micelles can also inhibit the transcriptional activity of the PI3K/Akt signaling pathway. Therefore, these findings provide new evidence for the pharmacological effects of mPEG-b-P(C7-co-CA) micelles.