Structural coupling between the horizontal and vertical tails in high-tailplane aircraft induces parasitic strain, complicates strain analysis, and potentially leads to inaccurate load estimations. Effective decoupling of parasitic strain is essential for obtaining the intrinsic vertical tail strain response, ensuring structural integrity assessment. This study proposed a strain decoupling framework that integrated ground calibration and flight test evaluation, employing the Least Absolute Shrinkage and Selection Operator (LASSO) for feature selection and regression sparsity. Ground calibration test applied controlled symmetric and asymmetric loading to the horizontal tail. This test developed a regression-based calibration model for estimating vertical tail parasitic strain. The proposed decoupling approach achieved a prediction error below 5% and an R2 exceeding 99.5%. Flight test was conducted to evaluate strain distribution responses under yaw and pitch maneuvers. Results indicated that parasitic strain could oppose or surpass intrinsic strain during certain flight conditions. These findings confirmed the accuracy of the strain decoupling method and provided new insights into flight strain transmission mechanisms, offering a robust foundation for structural analysis, aircraft design optimization, and improved load monitoring in high-tailplane configurations.