n-type single-crystal SnSe intrinsically possesses low lattice thermal conductivity and a multi-band electronic structure, while also being environmentally friendly, earth-abundant, and cost-effective. In this work, the effect of b-axis uniaxial strain ranging from -6% to 6% on the thermoelectric properties of n-type SnSe at 750 K is systematically investigated by combining first-principles calculations with Boltzmann transport theory. Both the constant relaxation time approximation (CRTA) and the energy-dependent relaxation time approximation (EDRTA) are employed to calculate the electrical transport parameters. The EDRTA, with its carrier-concentration-dependent relaxation time (τ ∝ n^-1/3) extracted from experimental data via the Seebeck coefficient calibration method, yields results in better agreement with experiments, as evidenced by a smaller root-mean-square deviation compared with CRTA. CRTA is found to significantly overestimate the power factor (PF) and the corresponding optimal carrier concentration. Within the EDRTA framework, a 2% tensile strain increases the maximum PF along the a-axis (PF_xx) from 19.7 to 22.1 μW cm^-1 K^-2, corresponding to an 11.9% enhancement, while a 4% tensile strain enhances the maximum PF along the b-axis (PF_yy) from 12.0 to 19.0 μW cm^-1 K^-2, corresponding to a 58.5% improvement. Band structure analysis reveals that tensile strain induces multi-conduction-band convergence. Under 2% and 4% tensile strains, the minimum energy offsets at the conduction band minimum are reduced to 0.00873 eV and 0.01601 eV, respectively, which are well below the energy convergence criterion ΔE < 2k_BT (≈ 0.13 eV at 750 K). Such convergence of the conduction bands leads to an increased density of states effective mass and an improved Seebeck coefficient. Consequently, under 4% tensile strain, the maximum thermoelectric figure of merit ZT reaches 3.2 along the a-axis and 1.6 along the b-axis, corresponding to 40.07% and 54.10% improvements over the unstrained case, respectively. The lattice thermal conductivity, estimated using the Slack model, decreases gradually with increasing tensile strain, further contributing to the ZT enhancement. This work elucidates the physical mechanism by which b-axis tensile strain improves the thermoelectric performance of n-type SnSe through multi-band convergence, and provides the optimal carrier concentration ranges for PF and ZT under various strain conditions: PF_xx peaks at carrier concentrations between 2.1 and 4.4×10²⁰ cm^-3, while ZT_xx reaches its maximum in the range of 2.1 to 7.8×10¹⁹ cm^-3, offering clear theoretical guidance for experimental optimization of SnSe thermoelectric performance via doping and strain engineering. This study demonstrates that b-axis tensile strain can effectively enhance the thermoelectric performance of n-type SnSe, thereby strengthening its potential for thermoelectric applications.