Precise diagnosis of lymph node metastasis is important for therapeutic regimen planning, prognosis analysis and probably better outcomes for cancer patients. In this work, Ga-labeled amphiphilic alternating copolymers nanoparticles with different rigid ligands were synthesized as positron emission tomography (PET) probes for lymph node metastasis imaging. The labeling efficiency and stability of nanoparticles was improved with increased rigidity of coordination unit. PU(Ga-L-MDI-PEG) nanoparticles (PU(Ga-L-MDI-PEG) NPs) with the strongest rigidity of coordination unit exhibited the lowest critical micelle concentration, the best Ga labeling efficiency and stability. During lymph node metastasis imaging, PU(Ga-L-MDI-PEG) NPs led to different accumulations in normal lymph nodes (N-LN) and tumor metastasized sentinel lymph nodes (T-SLN), which resulted in different PET signal presentation, making it feasible to differentiate N-LN from T-SLN. In comparison, small molecule probe GaL had poor lymph node accumulation, not only making it difficult to find lymph nodes on PET/computed tomography scan, but also tough to distinguish N-LN from metastatic ones. Overall, this work provides a reference for design of Ga labeled polymeric nanoparticles with high chelation efficiency and stability, as sensitive PET probes for lymph node imaging.