We synthesized amino-modified poly(ε-caprolactone) PCN-b-PEG-b-PCN (PECN) triblock copolymers and studied the contribution of the introduced amino groups to the drug delivery efficiency of PECN nanoparticles (NPs) and their injectable thermosensitive hydrogels. PECN15 with an optimal amino group content was obtained. Firstly, the hydrophobic drug paclitaxel (PTX) was loaded into PECN15 up to 5.91% and formed PTX/PECN NPs 90 nm in size and with a slightly positive charge (7.3 mV). Furthermore, the injectable PTX/PECN NPs aqueous solution (25 wt%) at ambient temperature could undergo fast gelation at 37 °C and sustainedly release PTX/PECN NPs in 10 days. More importantly, compared with our previously reported PECT NPs, the PECN NPs without an increase in toxicity could improve the cell uptake and enhance intracellular drug release by responding to the acidic environment of the endosome. Thus, the PTX/PECN NPs presented a lower IC50 of 3.14 μg mL-1 than that of the PTX/PECT NPs (7.67 μg mL-1) and free PTX (4.65 μg mL-1). Moreover, through peritumoral injection, the PTX/PECNGel showed 94.27% inhibition rate of tumor growth on day 19, higher than PTX/PECTGel (72.28%) and Taxol® (47.03%). Therefore, the PECN NPs hydrogel provided a more effective injectable platform to enhance local cancer chemotherapy, and also provided the possibility of further functionalization by the reactive amino groups.