A multifunctional pH-sensitive polymeric nanoparticle system was developed for simultaneous tumor magnetic resonance imaging (MRI) and therapy. The nanoparticles were self-assembled using the multi-block polymer poly(lactic acid)-poly(ethylene glycol)-poly(l-lysine)-diethylenetriamine pentaacetic acid (PLA-PEG-PLL-DTPA) and the pH-sensitive material poly(l-histidine)-poly(ethylene glycol)-biotin (PLH-PEG-biotin). The anti-hepatocellular carcinoma (HCC) drug sorafenib was encapsulated inside the nanoparticles. Gd ions were chelated to the DTPA groups which were distributed on the nanoparticle surface. Biotinylated vascular endothelial growth factor receptor (VEGFR) antibodies were linked to the surface biotin groups of nanoparticles through the avidin linker to form the target pH-sensitive theranostic nanoparticles (TPTN). TPTN exhibited spherical or ellipsoidal shapes, uniform particle size distribution (181.4 ± 3.4 nm), positive zeta potential (14.95 ± 0.60 mV), high encapsulation efficiency (95.02 ± 1.47%) and drug loading (2.38 ± 0.04%). The pH-sensitive sorafenib release from TPTN was observed under different pH values (47.81% at pH = 7.4 and 99.32% at pH = 5.0, respectively). In cell cytotoxicity studies, TPTN showed similar antitumor effect against HepG2 cells compared to solubilized sorafenib solution after pre-incubation in acid PBS (pH = 5.0) for 1 h in vitro (P > 0.05). In in vivo anti-tumor studies, TPTN showed significantly higher antitumor effect in H22 tumor (VEGFR overexpressed cell line) bearing mice compared to the solubilized sorafenib solution (oral or i.v. administration) group (P < 0.05). In the MRI test, the T1 relaxivity value of TPTN was 17.300 mM(-1) s(-1) which was 3.6 times higher than Magnevist® (r1 = 4.8 mM(-1) s(-1)). As a positive contrast agent, TPTN exhibited higher resolution and longer imaging time (more than 90 min) in the MRI diagnosis of tumor-bearing mice compared to Magnevist® (more than 60 min). Furthermore, histological examination of TBN (blank TPTN, without sorafenib loaded) showed no visible tissue toxicity compared to normal saline. Thus, TPTN possessed dual-loading drugs and imaging agents, active targeting and pH-triggered drug release properties in one platform with good biocompatibility. All of these results indicated that TPTN was a promising theranostic carrier which could be a platform for the development of novel multifunctional theranostic agents.