The structural transitions in solution-cast film of poly [5,5'-bis(2-butyloctyl)-(2,2'-bithiophene)-4,4'-dicarboxylate-alt-5,5'-2,2'-bithiophene] (PDCBT) copolymer have been systematically studied by a combination of differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), in situ Fourier transform infrared spectroscopy (FTIR), UV-vis spectroscopy, and in situ-wide-angle X-ray scattering (WAXS). The glass transition temperature of the main chain as well as the melting and crystallization temperatures of the polymer were determined as 10, 238, and 221 degrees C, respectively. The out-of-plane deformation vibration of PDCBT C-beta-H groups in FTIR has been manifestly assigned for the first time. A broad endothermic peak between 30 and 120 degrees C was observed during DSC heating process and was attributed to the enthalpic relaxation of the twist glass transition, which resulted in a small negative effect on the power conversion efficiency (PCE) of solar cells. A reorganization process in the crystalline region was observed in WAXS upon heating, including the increase of grain size along the a-axis.