Perovskite solar cell has gained wide attention due to its excellent photoelectric performance and economical fabrication process, with high-efficiency perovskite solar cell could be achieved by optimizing film morphology. In this work, a kind of Lewis-bases, urea(U), was used as a novel additive in synergy with DMSO to regulate the crystallization processes. Adding slight amount of DMSO can obviously improve the reproducibility of device and optimize the spin-coating process. The mechanism of the action can be explained by infrared spectroscopy. Under the coordination of the lead ion that could accept electron pairs and the C=O bond in urea, introduction of urea in perovskite precursor solution to form MAI center dot PbI2 center dot DMSO-urea complex could significantly improve the photoelectric properties of the perovskite films. It formed a smooth and large grain-size perovskite film with no visible pinhole or crack found. Finally, the device fabricated with the incorporation of 50% urea presented a high performance with a PCE of 17.82%.