It is well-known that the horseradish peroxidase- (HRP-) mimicking DNAzyme, namely, hemin/G-quadruplex, can effectively catalyze the polymerization of aniline to form DNA-guided polyaniline. Meanwhile, polyaniline exhibits extraordinary electrical, electrochemical, and redox properties, as well as excellent SPR signal-enhancing ability. Herein, we report a novel ultrasensitive surface plasmon resonance (SPR) biosensor based on HRP-mimicking DNAzyme-catalyzed in situ formation of polyaniline for signal amplification, using bleomycin (BLM) as the proof-of-concept analyte. The recognition and the subsequent cleavage of DNA probe P1 by BLM switches off the hybridization between P1 and the G-rich DNA probe P2, resulting in less hemin/G-quadruplex complexes and reduced DNA-guided polyaniline deposition on the SPR Au disk surface. As compared to the case when BLM is absent, a significant shift in SPR angle is observed, which is dependent on the BLM concentration. Therefore, ultrasensitive SPR detection of the target BLM is realized, with a detection limit down to 0.35 pM, much lower than those reported in the literature. Moreover, the proposed SPR biosensor has been successfully applied for the detection of BLM spiked in human serum samples. The HRP-mimicking DNAzyme-catalyzed in situ polyaniline deposition and polyaniline-assisted signal amplification not only significantly improves the specificity and the sensitivity of the BLM assay but also allows the ultrasensitive detection of other biomolecules by simply changing the specific target recognition DNA sequences, thus providing a versatile SPR biosensing platform for the ultrasensitive detection of a variety of analytes and showing great potential for application in the fields of bioanalysis and clinical biomedicine.