The global sulfur cycle is largely influenced by the production of dimethyl sulfide (DMS), which is primarily generated through the enzymatic cleavage of algal dimethylsulfoniopropionate (DMSP). This study presents an efficient and simplified method for analyzing DMSP lyase activity (DLA) by measuring the conversion efficiency of DMSP to DMS using selected ion flow tube mass spectrometry (SIFT-MS) coupled with a dissolved gas extraction device. Unlike conventional methods, which involve multiple steps such as trapping, desorption, and chromatographic separation, the proposed method consists of two streamlined steps: (1) Introduction of excess DMSP substrate into the sample vial, followed by continuous measurement of DMS evolution via SIFT-MS; (2) recording the DMS response at 3-s intervals and calculating the DMS production rate by dividing the integrated DMS mass over time intervals. The high-frequency detection of trace-level DMS enhances the accuracy of release rate measurements and aids in optimizing the DMSP substrate concentration. The performance of the proposed method was evaluated using cultured phytoplankton and natural seawater samples, achieving an analytical precision less than 10 % and a total analysis time of under 10 min, substantially faster than traditional gas chromatography-based techniques. This method provides a robust tool for investigating the dynamics of DMS-related processes in marine environments.