Abstract: In the area of “carbon peaking and carbon neutralization,” changing energy structure from primary energy to new energy is an extremely important issue. Due to the intermittent and fluctuating characteristics of new energy, energy storage technology has proven a viable solution to this issue thus has attracted extensive attention. As a key to energy storage technology, the problem of the low thermal conductivity of phase change materials (PCMs) requires immediate attention. Erythritol is a high enthalpy phase change material commonly used in low-to-medium temperature processes. Its thermal conductivity of only 0.7 W·m^–1·K^–1 seriously hinders its energy utilization efficiency in practical application. In this paper, erythritol is the main research focus, and single-walled carbon nanotubes (CNTs) of ultra-high thermal conductivity are used as thermal conductivity reinforcements. The effects of length, mass fraction, and the distribution of CNTs on the thermal conductivity of erythritol/CNT composite PCMs were studied by means of molecular dynamics simulation. When the axial lengths of the CNTs were less than their phonon mean free paths, the thermal conductivity of the composite PCMs increased with increasing CNT axial length and mass fraction, although clear anisotropy was exhibited. Due to the introduction of erythritol–CNTs interfacial thermal resistance, the radial thermal conductivity of the composite materials was lower than that of pure erythritol. When CNTs were randomly distributed in erythritol, the anisotropy of thermal conductivity was significantly improved, as was thermal conductivity in all directions. Comparing the phonon vibration densities of the states of erythritol and CNT before and after recombination, it was found that, due to the interaction between the two, the phonon vibration of CNT was suppressed, and the phonon heat transport in erythritol was excited, thus improving thermal conductivity.