The ultrahigh thermoelectric performance of SnSe-based single crystals has attracted considerable interest in their polycrystalline counterparts. However, the temperature-dependent structural transition in SnSe-based thermoelectric materials and its relationship with their thermoelectric performance are not fully investigated and understood. In this work, nanolaminar SnSe polycrystals are prepared and characterized in situ using neutron and synchrotron powder diffraction measurements at various temperatures. Rietveld refinement results indicate that there is a complete inter-orthorhombic evolution from Pnma to Cmcm by a series of layer slips and stretches along the a- and b-axes over a 200 K temperature range. This phase transition leads to drastic enhancement of the carrier concentration and phonon scattering above 600 K. Moreover, the unique nanolaminar structure effectively enhances the carrier mobility of SnSe. Their grain and layer boundaries further improve the phonon scattering. These favorable factors result in a high ZT of 1.0 at 773 K for pristine SnSe polycrystals. The thermoelectric performances of polycrystalline SnSe are further improved by p-type and n-type dopants (i.e., doped with Ag and SnCl2, respectively), and new records of ZT are achieved in Ag0.015Sn0.985Se (ZT of 1.3 at 773 K) and SnSe0.985Cl0.015 (ZT of 1.1 at 773 K) polycrystals.