In the present study, we devised a green-synthesis route to NaYF4:Gd3+,Yb3+,Er3+ upconversion nanoparticles (UCNPs) by using eco-friendly paraffin liquid, instead of 1-octadecene, as a high boiling non-coordinating solvent. A multifunctional nanoplatform was then developed by coating UCNPs with biocompatible transferrin (TRF) for magnetically-assisted and near-infrared light induced photodynamic therapy and bioimaging. Protoporphyrin IX (PpIX), a clinically approved photodynamic therapy agent, was loaded into the shell layer of the TRF-coated UCNPs (UCNP@TRF nanoparticles), which can be efficiently taken up by cancer cells for photodynamic therapy. Upon near-infrared light irradiation, the UCNP@TRF-PpIX nanoparticles could not only kill the cancer cells via photodynamic therapy but also serve as imaging probes. We also demonstrated that an external magnetic field could be used to increase the uptake of UCNP@TRF-PpIX nanoparticles by MDA-MB-231 and HeLa cancer cells, and hence result in an enhanced photodynamic therapy efficiency. This work demonstrates the innovative design and development of high-performance multifunctional PDT agents.