Ahigh-performance plunger pump, as the heart of hydraulic equipment, provides a prerequisite for safe and economical operation. Ultrahigh-pressure plunger pumps are thorn in the throat of China's equipment manufacturing industry. As a critical component of the plunger pump, achieving the localization of its material, securing superior manufacturing performance, and enabling cost control has become the paramount priority at present. Taking special horizontal continuous casting LZQT600-3 ductile iron dens bars for the plunger pump cylinder as the research object, the microstructure characteristics of different parts (surface, 1/2R,center) were compared and analysed, and the carbonitriding parameters of the samples in different parts were optimized through orthogonal experiments. The results show that the spheroidization level and the number of graphite spheres decrease successively from the surface and 1/2R to the center of the dens bars, which are 554, 509 and 492 per mm2, respectively; the average diameters of the graphite spheres are 29.86, 26.78 and 31.03 μm, respectively; the quantities of pearlite in the matrix are 64.4%, 65.9% and 70.8%; and the lamellar spacings of pearlite are 274, 286 and 316 nm, respectively. The optimal carbonitriding parameter scheme obtained on the basis of the best wear resistance requirements of the plunger pump cylinder is 580 ℃×4h, and the NH3∶CO2 ratio is 4∶3. The diffusion layer structure is composed of an oxide layer, a compound layer composed of the ε-Fe2-3(N, C) phase and γ′-Fe4N phase, and a diffusion layer composed of the α-Fe phase.