To enhance the accuracy of fatigue life prediction for BFRP tendons under combined environmental and loading conditions, fatigue tests were conducted on BFRP tendons at different stress ratios. The failure modes and S-N curves were obtained, and the evolution mechanism of failure modes with varying stress ratios was elucidated: transitioning from “fiber breakage” dominance at low stress ratios to “interfacial debonding and wear” dominance at high stress ratios. Five typical constant life diagram (CLD) models were evaluated, and an improved fatigue life prediction method was proposed. Through theoretical derivation, a two-dimensional CLD model was integrated with the power function model of S-N curves to construct a three-dimensional constant life surface characterizing the relationships among fatigue life, stress amplitude, and mean stress. The results demonstrate that under low stress ratio conditions where fiber breakage dominates the failure, the prediction error of this model ranges from 1.37% to 10.1%, achieving a high level of accuracy.