Oxidative toxicity has impeded the development of haemoglobin-based oxygen carriers (HBOCs) by causing methaemoglobin (MetHb) formation and inducing oxidative stress. In our previous work, polydopamine-coated haemoglobin (Hb-PDA) nanoparticles have been designed and synthesized with the capacity to reduce oxidative toxicity. In this investigation, the mass ratio of dopamine (DA) to haemoglobin (Hb) and the pH value are found to be the primary factors that influence preparation of Hb-PDA nanoparticles. X-ray photoelectron spectroscopy showed that the catechol groups of DA play a crucial role in the modification of Hb surface. Hb-PDA nanoparticles were found to exhibit oxidative protection from hydrogen peroxide (H2O2) and the change of mitochondrial membrane potential showed that the Hb-PDA nanoparticles reduced H2O2-induced apoptosis. It is demonstrated that modification of PDA could maintain the oxygen-release capacity of Hb. These findings confirm that Hb-PDA nanoparticles possess restrained oxidative toxicity and preserve oxygen-release capacity.