The introduction of solvent additives is one of the most common approaches for enhancing the power conversion efficiency of organic solar cells(OSCs). However, the use of solvent additives has some negative effects,and an understanding of how solvent additives affect OSCs is currently limited. In this study, we developed an in situ grazing incidence wide-angle X-ray scattering(GIWAXS)technique in the SAXS beamline(BL16 B1) at the Shanghai Synchrotron Radiation Facility, and the additive effects of1,8-diiodoctane(DIO) on the performance and morphology evolution of the PTB7-Th/PC71 BM device was investigated in depth. The results revealed that the crystal size increased with the volume ratio of DIO, and a drastic evolution of lattice space and crystal coherence length was observed during thermal annealing for the first time, to our knowledge. The discrete PC_(71)BM molecules dissolved by DIO have an effect similar to that of the nucleating agent for PTB7-Th, boosting the crystallization of PTB7-Th,reducing phase separation, and inducing more drastic morphological evolution during thermal annealing. Our results provide a deep perspective for the mechanism of solvent additives, while also showing the significance and feasibility of the in situ GIWAXS technique we developed at BL16 B1.