Four MnO2 samples were synthesized through a simple reaction of KMnO4 with high-purity graphite in different concentrations of sulfuric acid at low temperature. Their morphology, crystal structure and performance as cathode catalysts of zinc-air battery were investigated with X-ray diffraction (XRD), Fourier transformation infrared spectrometer (FTIR), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) and electrochemical tests. It is found that the crystal structure and the morphology of the synthesized samples depend on the sulfuric acid concentration. The synthesized samples have large specific surface area and exhibit excellent performance compared to the commercial electrolytic manganese dioxide (γ-MnO2). Two varieties of manganese dioxides, δ-MnO2 (the as-prepared samples a and b) and α-MnO2 (the as-prepared sample c and d), were obtained when using low and high sulfuric acid concentration, respectively. Higher sulfuric acid concentration favors the agglomeration of the particles. The specific surface area of samples a, b, c, d, and γ-MnO2 is 83.2, 81.1, 88.5, 86.8, and 43.5m2 g−1, corresponding to discharge capacity of zinc-air batteries is 169.5, 160.3, 175.2, 171.5, and 112.2mAh, respectively.