Design of multifunctional nanoplatforms combined with ultrasound-targeted microbubble destruction (UTMD) technology for enhanced tumor accumulation is feasible to solve the bottleneck of theranostics. Herein, we present the development of zwitterion-modified gadolinium (Gd)-chelated core-shell tecto dendrimers (CSTDs) as a nanomedicine platform (PCSTD-Gd) for enhanced magnetic resonance (MR) imaging-guided chemo-gene therapy of orthotopic breast cancer with the assistance of UTMD. In our design, CSTDs synthesized via supramolecular recognition of β-cyclodextrin and adamantane were covalently linked with tetraazacyclododecane tetraacetic acid-Gd(III) chelators, modified with 1,3-propane sultone to achieve good protein-resistance property, and used for co-delivery of an microRNA 21 inhibitor (miR 21i) and an anticancer drug doxorubicin (DOX). The overall design is quite advantageous and cooperative. The CSTDs with a greater size than single-generation core dendrimers have amplified the enhanced permeability and retention effect for better passive tumor targeting, with a larger r1 relaxivity for sensitive MR imaging and serum-enhanced gene delivery efficiency due to the better compaction ability as well as the protein resistance ability, and with larger interior space for improved drug loading. Through the unique design and the assistance of UTMD, the obtained PCSTD-Gd/DOX/miR 21i polyplexes enable enhanced MR imaging-guided combined chemo-gene therapy of an orthotopic breast cancer model in vivo.