NIR-II (1000-1700 nm) fluorescence imaging is continually attracting strong research interest. However, current NIR-II imaging materials are limited to small molecules with fast blood clearance and inorganic nanomaterials and organic conjugated polymers of poor biodegradability and low biocompatibility. Here, we report a highly biodegradable polyester carrying tandem NIR-II fluorophores as a promising alternative. The polymer encapsulated a platinum intercalator (56MESS, (5,6-dimethyl-1,10-phenanthroline) (1S,2S-diaminocyclohexane) platinum(II)) and was conjugated with both a cell-targeting RGD peptide and a caspase-3 cleavable peptide probe to form nanoparticles for simultaneous NIR-II and apoptosis imaging. In vitro, the nanoparticles were approximately 4-1000- and 1.5-10-fold more potent than cisplatin and 56MESS, respectively. Moreover, in vivo, they significantly inhibited tumor growth on a multidrug-resistant patient-derived mouse model (PDXMDR). Finally, through label-free laser desorption-ionization mass spectrometry imaging (MALDI-MSI), in situ 56MESS release in the deeper tumors was observed. This work highlighted the use of biodegradable NIR-II polymers for monitoring drugs in vivo and therapeutic effect feedback in real-time.