Abstract: To address the nonuniform melting and delayed melting at the far-end bottom region of composite phase change materials under constant-temperature sidewall heating, a three-dimensional transient numerical model was developed for a paraffin-based composite phase change material incorporating a continuous-gradient body-centered cubic (BCC) metallic lattice. Three continuous-gradient structures, N05-BCC, N10-BCC, and N20-BCC, were constructed with the same overall average porosity and local porosity near the heated wall. The effects of the porosity distribution along the heat-flow direction on melting characteristics were investigated and compared with those of a uniform BCC structure (U-BCC). The results show that the porosity distribution has only a minor effect during the early melting stage, whereas it mainly influences the middle and late stages by regulating the heat-conduction paths of the metallic lattice and the flow space available to the liquid phase. The complete melting time of N10-BCC/PCM is 5314.5 s, which is 23.69% shorter than that of U-BCC/PCM. Moreover, among the three graded structures, N10-BCC/PCM exhibits the earliest phase-change completion and the shortest local phase-change duration at the far-end bottom monitoring point. These results indicate that the enhancement achieved by continuous-gradient structures depends on the balance between metallic heat-conduction pathways and liquid-flow space established by the spatial porosity distribution, rather than simply on increasing the gradient exponent.