The morphology of perovskite layer is a deterministic factor in determining photovoltaic performance of perovskite solar cells (PSCs). In this paper, by using two-step spin-coating method, different electron donating ability solvents including DMF, DMF + CH3COCH3, DMF + DMSO + HAc, DMSO + HAc and DMSO + ACN were selected to investigate the nucleation and growth mechanism of PbI2 adduct and perovskite film by X-ray diffraction (XRD), scanning electron microscopy (SEM) and time-resolved-photoluminesence (TRPL). The solution DMSO + ACN was chosen as the optimal solvent due to its higher performance of the constructed device. Moreover, the effect of volume ratio of DMSO to ACN (V-DMSO/(ACN)) on the perovskite film was systematically studied. The results indicated that more compact perovskite film with larger grain size could be formed with ACN amount increasing, which led to higher power conversion efficiency (PCE) of the solar cell. Dynamic light scattering (DLS) analysis of PbI2/DMSO + ACN solutions was used to elucidate the phenomenon. With ACN amount increasing from 19:1 to 7:3, the radius of ACN-PbI2-DMSO clusters in solutions decreased from 694.2 to 365.1 nm, resulting in formation of porous PbI2 adduct films, thus providing MAI solution more transport channels and adequate space for grain growth. Under the air relative humidity of 30%, the device PCE of 15.32% was achieved at the V-DMSO/(ACN) of 7:3.