In digital holographic particle field imaging, the small aperture angle of particle diffraction results in an increased depth of focus during reconstruction. This leads to a significantly lower axial positioning accuracy compared with lateral positioning accuracy. Therefore, higher axial positioning accuracy can be achieved by increasing the illumination wavelength, which is equivalent to increasing the particle aperture angle. This study proposes the use of infrared coherent light source to illuminate the particle field to improve the axial positioning accuracy of digital holographic particle field reconstruction without increasing the complexity of algorithms and systems. This study theoretically analyzes the relationship between focal depth and axial positioning accuracy in digital holographic particle field reconstruction. Simulation and analysis of holographic particle field reconstruction are conducted under green, red, and infrared light illumination. Moreover, holographic imaging experiments of polystyrene microsphere particle field based on these three light sources are performed. The simulation and experimental results show that compared with red and green lights, the infrared light source reduces the focal depth by approximately 19% and 39%, respectively. Also, increasing the wavelength weakens the interlayer interference of defocused images, thereby improving axial positioning accuracy.