Abstract: Objective: To quantitatively evaluate the relative contributions of the four stroke segments to the performance improvement of national master level men’s 200 m individual medley; to clarify the effect of high-altitude environment combined with explosive power training on stroke efficiency during breaststroke segment; to test the mediating effect of dry-land strength performance between training factors and stroke efficiency during breaststroke segment. Methods: A randomized block crossover design was adopted. A total of 6 male national master level athletes specializing in the 200 m individual medley were enrolled and randomly divided into two groups stratified by their best performance in the past 1 year. Both groups completed four 5-week intervention periods separated by one-week washout intervals. Swimming training structure and volume as well as the duration and energy expenditure of dry-land training, remained consistent across all intervention periods. The four training conditions were flatland environment combined with regular bodyweight training, flatland environment combined with explosive power training, high-altitude environment combined with regular bodyweight training, and high-altitude environment combined with explosive power training. The altitude was 1,888 m in high-altitude training. At baseline and after each intervention period, 200 m individual medley performance, breaststroke segment stroking efficiency, dry-land strength performance, body composition, and physiological and biochemical indicators were measured. Fixed-effect regression, Pratt relative importance analysis, and dual-level Bayesian structural equation modeling were used to examine the contribution of each swimming segment and the path relationships among training factors, dry-land strength performance, and breaststroke segment stroking efficiency. Results: All participants achieved improvements in 200 m individual medley performance, among which the optimization of breaststroke segment contributed most to the overall performance improvement. Under identical dry-land training mode, high-altitude environment contributed more to the improvement of breaststroke segment stroking efficiency. Moreover, compared with regular bodyweight training, explosive power training presented a superior tendency in improving breaststroke segment stroking efficiency. Path analysis further showed that the improvement of comprehensive dry-land strength performance was positively associated with improvements in breaststroke segment stroking efficiency, which was mainly manifested as a direct effect. The improvement in breaststroke segment stroking efficiency may be associated with the direct effect induced by the synergistic effect of explosive power training and high-altitude environment. In addition, the indirect effect of changes in dry-land strength performance caused by training environment, training mode, and their synergistic effect on breaststroke segment stroking efficiency was not significant, suggesting that the transfer effect of dry-land training may be affected by the match between training content and sport-specific demands, rather than merely depending on marked changes in dry-land strength performance indicators. Conclusions: Improving breaststroke segment performance makes a relatively high contribution to better performance of national master level men’s 200 m individual medley; high-altitude environment combined with explosive power training improves breaststroke segment stroking efficiency, which is more likely caused by the direct effect of the synergistic effect of explosive power training and high-altitude environment, rather than the stable mediation effect caused by improved overall dry-land strength performance.