Sugarcane is a globally important cash crop and bioenergy crop, playing a critical role in promoting economic development in sugarcane-producing countries. Phosphorus is an essential nutrient for plants, and low phosphorus availability in soil often constrains the quality and yield of sugarcane in China. S. officinarum serves as an important genetic foundation for modern sugarcane varieties. Elucidating the low-phosphorus tolerance mechanisms in phosphorus-efficient genotypes of S. officinarum is of significant theoretical and practical importance for screening and breeding phosphorus-efficient sugarcane varieties, improving phosphorus fertilizer utilization efficiency, and promoting the development of the sugarcane industry. The study was aimed to explore the response characteristics of different S. officinarum genotypes under low-phosphorus stress from the perspectives of root morphology and physiological adaptations, identify key factors affecting phosphorus absorption, and provide a theoretical basis for improving nutrient utilization efficiency in sugarcane through root plasticity modification. Phosphorus-efficient S. officinarum genotypes (48 Mouna and Loether) and phosphorus-inefficient genotypes (NC20 and Badila) were used in the study. Under two phosphorus supply conditions, low phosphorus (0.2 μmol/L KH₂PO₄) and normal phosphorus (1000 μmol/L KH₂PO₄), comparative analyses were conducted on root morphological indices, low-phosphorus tolerance indices, root activity, acid phosphatase activity, and other physiological characteristics of each variety. The results indicated that low-phosphorus stress could inhibit the growth and development of various sugarcane tropical varieties to different degrees. Among them, the phosphorus-efficient variety Loether was the least affected by low-phosphorus stress in terms of plant height, stem dry weight, and whole-plant biomass, while its root dry weight and root-to-shoot ratio significantly increased. Low-phosphorus stress promoted increases in total root length, total root surface area, and total root volume in both Loether and 48 Mouna, with Loether exhibiting the most pronounced increase. The specific root length, specific root surface area, and specific root volume of most varieties decreased, with NC20 showing the greatest reduction. Additionally, all varieties exhibited root thinning. Furthermore, under low-phosphorus stress, 48 Mouna and Loether demonstrated higher root low-phosphorus tolerance indices, root activity, and phosphorus acquisition capacity. In conclusion, this study reveals that the phosphorus-efficient tropical varieties 48 Mouna and Loether can better optimize root architecture, enhance phosphorus absorption interfaces, maintain strong root activity and enzymatic adaptability, effectively improve phosphorus absorption and utilization efficiency, and exhibit strong adaptability to low-phosphorus stress.