Ribosome-inactivating proteins (RIPs) are potent protein toxins for cancer therapy, and they have strong ability to inhibit protein synthesis and induce cell death via inactivation of ribosomes in eukaryotic cells. However, the delivery of RIPs has been a challenging task due to their large molecular weight and lack of targeting property. Low molecular weight protamine (LMWP), a transmembrane peptide, has been proved to effectively promote transmembrane transportation, whereas the enzyme-activatable system can enhance the specificity by enhancing the tumor drug concentration through enzymatic reaction. We herein constructed a self-assembly and stimuli-responsive fusion gelonin delivery system. Gelonin, a typical RIP protein, was assembled with nickel ferrite nanoparticles by self-assembling between hexa-histidine tag (His-tagged) and nickel ions. Both in vitro and in vivo results indicated that the magnetic nanoparticle carriers and the applied linkers did not damage the pharmaceutical effect of gelonin, and the whole drug delivery system showed good biocompatibility, sensitive selectivity, and significantly enhanced cytotoxic activity. This in turn presented theranostic nanoparticles as efficient delivery vehicle for clinical use.