Non-axisymmetric guided wave can determine a defect’s axial and circumferential positions within a circular pipe through its unevenly distributed energy distribution. Nevertheless, low-frequency and low-order non-axisymmetric guided waves exhibit rapid changes in energy distribution, rendering them unsuitable for defect localization in long straight circular pipes. This paper proposed a defect detection and localization method in long straight aluminum circular pipes by using the high-order non-axisymmetric guided wave mode L(M,5) at 1.6 MHz. Initially, finite element simulations were employed to excite the high-order L(M,5) mode within an aluminum cylindrical pipe, facilitating a quantitative analysis and validation of the L(M,5) mode’s energy distribution across various axial positions. Subsequently, based on the defect-induced non-axisymmetric guided wave model, the superiority of the higher-order mode in defect localization was illustrated when compared to its lower-order counterpart. The results indicated that the low-order non-axisymmetric guided wave mode L(M,2) can only perform defect localization within a relatively short distance. In contrast, the L(M,5) mode, with a longer period of energy distribution changing, can conduct defect identification and localization over a longer distance.