The work aims to prepare a surface diffusion coating on Ti-6Al-4V alloy by means of diffusion treatment on the titanium alloy surface through chemical heat treatment, and improve cavitation erosion resistance of the alloy. The surface of annealed Ti-6Al-4V alloy sample was treated by aluminizing in nitrogen atmosphere to obtain a hard surface infiltration layer. Phase constituents, microstructure, element distribution and mechanical properties of the diffusion coating were characterized by X-ray diffraction, scanning electron microscope, energy dispersive spectroscopy and Vickers microhardness tests. Subsequently, cavitation erosion tests were performed by an ultrasound vibration testing apparatus in accordance with ASTM G32 standard to explore the cavitation erosion resistance of the diffusion coating in combination with the surface morphology and element distribution after cavitation erosion. The results indicated that the diffusion treatment produced surface infiltration layer with higher microhardness on the surface of Ti-6Al-4V alloy. The coating consisted of the outermost compound layer and the inner diffusion layer. After 1 h of treatment at 1023 K, its surface infiltration layer was dense, uniform and microcrack-free. After 12 h of cavitation, the specimen treated at 1023 K for 1 h had the least cumulative volume loss was 18% of that of the untreated sample, so it had the best cavitation resistance. With the increase of treatment time and temperature, the surface of the diffusion coating with the hard ceramic particles became uneven and there were more and more hard ceramic particles protruding from the surface. The hard ceramic particles could significantly increase the surface microhardness of the treated specimen, but it was likely to induce the formation of cavitation and reduction of cavitation resistance. In summary, aluminizing treatment in nitrogen atmosphere can considerably improve the cavitation erosion behavior of Ti-6Al-4V alloy. The hard ceramic particles with uneven size protruding from the surface weaken the cavitation erosion resistance of the surface infiltration layer.