Design of simple-component theranostic agents with enhanced tumor accumulation represents a novel strategy in the area of precision nanomedicine for effective tumor theranostics. Herein, we report an innovative theranostic strategy based on ultrasound-enhanced tumor accumulation of a theranostic agent of phosphorus dendrimer-copper(II) complexes (1G3-Cu). The employed 1G3-Cu complexes represent a drug-free nanotheranostic agent with a simple composition possessing an r1 relaxivity of 0.7024 mM−1 s−1 for T1-weighted magnetic resonance (MR) imaging and an IC50 of 1.24 μM to inhibit the growth of a pancreatic cancer cell line. We show that cancer cells can be inhibited by the 1G3-Cu complexes through the activation of the apoptotic process with the upregulation of Bax, P53 and PTEN, downregulation of Bcl-2, and decrease of intracellular ATP. With the assistance of ultrasound targeted microbubble destruction (UTMD) technology that can increase cell membrane permeability temporarily via sonoporation effect, the 1G3-Cu complexes enable enhanced tumor MR imaging and chemotherapy due to their improved tumor accumulation. With the proven minimal toxicity to normal tissues at imaging and therapeutic doses, the prepared 1G3-Cu complexes may be potentially employed for UTMD-enhanced theranostics of different types of cancer.