Bi₂Te₃-based compounds have garnered significant attention for near-room-temperature thermoelectric applications due to their excellent electrical transport properties and low thermal conductivity. Solid solutions and doping are effective methods for optimizing the performance of Bi₂Te₃-based materials. Currently, n-type Bi₂Te_3-xSe_x materials using selenium (Se) as a dopant have been reported. However, the regulatory mechanisms of direct Se doping at Te sites on their defect structure, microstructure, and bandgap have not yet been systematically investigated. This work systematically investigates the regulatory behavior of direct Se doping at Te sites on the defect structure, microstructure, and bandgap of ternary n-type Bi₂Te_3-xSe_x compounds, and its impact on thermoelectric transport properties. Se substitution at Te sites forms n-type donor defects Se_Te^·, suppresses the formation of Bi'_Te antisite defects, and facilitates the return of Bi atoms to their intrinsic lattice sites. Concurrently, it introduces Te interstitial atoms (Te_i^×) and Te vacancies (V_Te^··), optimizing both carrier concentration and mobility, thereby effectively enhancing the electrical performance. Furthermore, supersaturated Te diffuses out as interstitial atoms and precipitates to form secondary phases. Se doping enhances phonon scattering via mass and strain field fluctuations induced by point defects, leading to a significant reduction in lattice thermal conductivity. As x increases, the bandgap of the samples is widened, resulting in significant suppression of the performance degradation caused by the intrinsic-excitation-induced bipolar effect. Consequently, the Bi₂Te_2.7Se_0.3 sample achieved amaximum average zT (zT_ave) value of 0.73 within the 300-500 K temperature range. After annealing, the optimization of the sample's microstructure led to an enhanced power factor and reduced thermal conductivity in the Bi₂Te_2.4Se_0.6 sample, achieving a maximum zT value of 0.81 at 420 K and and a zT_ave value of 0.76 in the 300-500 K temperature range. These results demonstrate that direct Se doping at Te sites can broaden the temperature range corresponding to the optimal zT values, and that the annealing process can further optimize the thermoelectric performance. This study provides significant insights for developing high-performance near-room-temperature thermoelectric materials applicable to broad operating temperatures.