Developing simple methods to synthesize perovskite layers under ambient air can facilitate the industrial production of perovskite solar cells (PSCs). While limited progress has been made for the ambient-air fabrication of formamidinium lead triiodide perovskite (alpha-FAPbI(3)) layers due to the coexisting non-perovskite polymorph (delta-FAPbI(3)). Here, a facile N-methyl pyrrolidone (NMP) additive-based method is proposed to produce FA-perovskite (FAPbI(3)) layers in ambient air and smooth, highly-crystallized and delta-phase free perovskite films are produced under humidity of similar to 40%. The resultant PSC delivers a power conversion efficiency (PCE) of 17.29%, similar to 20% enhancement from that of PSC produced from traditional dimethyl sulfoxide (DSMO) additive. It is found that a distinct FAI center dot PbI2 center dot NMP intermediate phase forms during the film growth, providing an alternative crystallization process different to that of traditional delta-FAPbI(3) intermediate route. The fine nucleation of FAI center dot PbI2 center dot NMP intermediate contributes a smooth and dense morphology to the final film. The rapid detachment of NMP molecule from the intermediate enables a direct formation of alpha-FAPbI(3) film, averting the incomplete delta- to alpha- phase conversion under ambient air. Due to the high film quality, a PCE of 13.55% is still remained after 30-day storage for unencapsulated device, demonstrating the reliability of our method for FA-perovsklte ambient-air fabrication.