To investigate the variation in total sulfur (TS) content in plant-soil systems of Phragmites australis and Cyperus malaccensis in the process of their spatial expansion in Shanyutan of the Min River estuary, the P. australis marsh (PM), C. malaccensis marsh (CM), and their ecotone (P. australis-C. malaccensis marsh, PCM) were studied in July 2015. Results showed that the TS content in soils of different marshes were generally in the order of CM > PCM > PM and the differences among them were primarily related to the alteration of soil texture and organic matter caused by the spatial expansion of P. australis and C. malaccensis, which substantially influenced the S transfer and transformation processes. The horizontal and vertical variation in TS content in the PCM soils was significantly different from those in the PM or CM community, and this difference was caused by the root distribution of different plants in the ecotone and their competition for the S nutrient. The TS content in soils of the PCM was more affected by the root distribution of P. australis, and TS content and stocks were generally higher in deep soil. The spatial expansion greatly altered the height, density, and biomass allocation of the two plants, and although their allocation patterns of belowground biomass in the ecotone were generally higher than those in the pure communities, P. australis exhibited a higher occupancy capacity in belowground space than did C. malaccensis. This study determined that the space expansion of P. australis and C. malaccensis was bi-directional, and could adapt to the competitive environment by using different strategies for S uptake, accumulation, and allocation. P. australis in the ecotone competed primarily by increasing the S accumulation capacity of its roots, whereas C. malaccensis in the ecotone was able to resist the space expansion of P. australis by expanding in the aboveground space and increasing the S accumulation capacity of aboveground organs.