Paper spray ionization mass spectrometry (PSI MS) has emerged as a notable method for the rapid analysis of biological samples. However, the typical cellulose-based paper tip is incompatible with protein detection due to the strong interaction between cellulose hydroxyl groups and proteins. In this study, we utilized a commercially available polyolefin-based synthetic paper, Teslin^®, as an alternative PSI substrate for simple protein analysis. We have named this method "droplet PSI" MS, as the aqueous protein solution droplet retains its shape on the Teslin^® paper tip. For droplet PSI, no further chemical pretreatment was necessary for the Teslin^® substrate; the only required preparation was shaping the Teslin^® paper into a triangular tip. In droplet PSI MS, protein ion signals were instantly detected from a protein solution droplet upon applying a spray solvent in situ along with high voltage (HV). When compared with conventional PSI MS, our method demonstrated superior sensitivity. The droplet PSI MS utilizing Teslin^® also showcased flexibility in real-time observation of protein alterations induced by an acid additive. Additionally, the effects of spray solvent composition and the application method were discussed.