Objective: This study aimed to explore the efficacy and optimal delivery time of human umbilical cord mesenchymal stem cells (hUC-MSCs) in treating collagenase-induced Achilles tendinopathy. Methods: Achilles tendinopathy in rats at early or advanced stages was induced by injecting collagenase I into bilateral Achilles tendons. A total of28 injured rats were injected with a hUC-MSCsolution or normal saline into bilateral tendons twice andsampled after 4weeks for histological staining, gene expression analysis, transmission electron microscope assayand biomechanical testing analysis. Results: The results revealed better histological performance and a larger collagen fiber diameter in theMSC group. mRNA expressionof TNF-α, IL-1βand MMP-3 was lower after MSC transplantation. Early MSC delivery promoted collagen I and TIMP-3 synthesis, and strengthened tendon toughness. Conclusion: hUC-MSCs demonstrated atherapeutic effect in treating collagenase-induced Achilles tendinopathy, particularly in the early stage of tendinopathy.