Employing additive to regulate the morphology of perovskite film is an effective method to enhance both the power conversion efficiency and long term stability of organic-inorganic hybrid perovskite solar cells. Here, we demonstrate that guanidinium thiocyanate (GuSCN) is a suitable additive for methylammonium lead iodide (MAPbI(3)) perovskite materials. Addition of GuSCN into MAPbI(3) can simultaneously enhance the crystallinity, enlarge the crystal size, and reduce the trap density of the perovskite films. As a result, the MAPbI(3) perovskite with 10% GuSCN exhibits superior power conversion efficiency of 16.70% compared to the pristine MAPbI(3) perovskite solar cell (15.57%). At the same time, the MAPbI(3) perovskite solar cell with GuSCN additive shows better stability, power conversion efficiency retains similar to 90% of its initial value compared to only 60% for pristine MAPbI(3) perovskite solar cells after being stored for 15 days without encapsulation.