Band structure modification plays an important role in improving the thermoelectric performance of SnTe. Herein band sharpening as one of the band structure modifications is achieved by Cl doping and reduces the effective mass (from 0.168m_e to ∼0.06m_e), which introduces a super high carrier mobility (over 2000 cm² V⁻¹ s⁻¹) and in turn optimizes the electrical conductivity but deteriorates the Seebeck coefficient. However, the numerical calculation based on the two-band model reveals that the Seebeck coefficient recovers to a normal level close to that of pristine SnTe by temperature-driven band convergence. As a result, a maximum power factor of ∼21 μW cm⁻¹ K⁻² at 873 K is achieved in the SnTe_0.94Cl_0.06 sample, approximately 30% higher than that of pristine SnTe. Furthermore, numerous micro-nano precipitates are observed in Cl-doped samples, giving rise to a drastic temperature dependence of the lattice thermal conductivity, thereby leading to an ultralow lattice thermal conductivity of ∼0.31 W m⁻¹ K⁻¹ at 523 K in the SnTe_0.88Cl_0.12 sample which is close to the Born–von Kaman periodic boundary conditions for the minimum limit. Unexpectedly, the Cl doping activates an obvious bipolar effect at high temperature. In spite of the bipolar effect contributing to the total thermal conductivity, the figure of merit ZT is still improved from ∼0.4 for pristine SnTe to ∼0.78 for the SnTe_0.88Cl_0.12 sample at 873 K due to the high power factor.