To overcome the strong negative charge and improve the endocytosis of poly-β-malic acid (PMLA) as a drug carrier, a pH-sensitive nanoconjugate of PMLA/hyd-PEG5000/PEG2000-TAT/DOX (PHPTD) was developed. The trans activator of transcription (TAT) modified with polyethylene glycol2000(PEG2000) was conjugated with the PMLA backbone which improved the endocytosis of PMLA. PEG5000 was utilized to shield TAT by a pH-sensitive hydrazone (Hyd) bond. In order to decrease the potential risk of accelerated blood clearance (ABC) phenomenon by anti-PEG IgM, the minimal content of TAT for penetrating tumor cells and the optimal protecting layer density of PEG5000 were screened. The result showed that 0.3 mol% TAT was enough to efficiently improve cellular uptake of PMLA (30 kda). The cytotoxicity and the 1H-NMR results indicated that 3.6 mol% PEG5000-modified nanoconjugates could shield 0.3 mol% TAT. The antitumor effect in breast cancer cells (MDA-MB-231) in tumor-bearing BALB/C mice demonstrated that this nanoconjugates exhibits high therapeutic efficiency in artificial solid tumors and low toxicity to normal tissues. It is indicated that TAT could be hidden in the long chain of PEG5000 at a neutral pH, when arrival to the tumor extracellular microenvironment, PEG5000 was cleaved from the nanoconjugates through the hydrazone bond due to the acidic tumor environment. Then, TAT was exposed, allowing the nanoconjugates to be transported into tumor cells. Our findings provide important and detailed information regarding the optimal content of TAT and the shielded density of PEG5000 and reveal their abilities of tumor penetration and potential for the efficient drug carrier.