Nanoparticles developed for detecting tumors have typically been modified with targeting ligands and signal agents using complex and costly conjugation processes. But this approach can lead to an excess of ligands on the nanoparticle surface, and this causes nonspecific binding and aggregation of nanoparticles, which decreases detection sensitivity. Here, we show that easily synthesized magnetoferritin nanoparticles with an iron oxide core encapsulated in a recombinant human heavy-chain ferritin (HFn) can specifically target and visualize tumour tissues. The HFn protein shell binds specifically to tumour cells via overexpressed transferrin receptor 1 (TfR1s), while the iron oxide core exhibits peroxidase activity that visualizes the tumours in the presence of chromogen substrates. Based on this difunctional nanoparticle, we developed a new method for cancer diagnosis. It achieves tumors targeting and visualization in one step, avoiding multi-step incubations with expensive and unstable antibodies, and repeated washing procedures of immunohistochemistry. We examined 541 clinical specimens from patients with ten types of cancer and verified that this method can distinguish cancerous cells from normal cells with a sensitivity of 95.77% and specificity of 95.72%. This new magnetoferritin nanoparticle-based method is more rapid and simpler to implement and provides new ideas, new reagents and new technologies for cancer diagnosis and therapy.