New particle formation (NPF) has a great impact on air quality and global climate. However, owing to the lack of observation, the NPF mechanism in the relatively clean coastal urban environment is still unclear. To investigate the key species involved in the NPF mechanism, we conducted the field observations in combination with machine learning in coastal city of Xiamen in Southeast China. The frequency of NPF events occurred was around 42.9%, with the mean nucleation rate (J1.3) of 12.28 cm−3 s−1. The random forest model demonstrated that gaseous methanesulfonic acid (MSA) was the prominent contributor in promoting NPF. Moreover, the measured data displayed that a clear co-occurrence between MSA and the observed nucleation process, suggesting MSA have a crucial impact on driving nucleation, the first steps of NPF, in Xiamen. Our results provide a new insight into the role of MSA in urban coastal NPF events.