Crab species distributions in the tidal mudflat are affected by many environmental factors, such as sediment parameters and mangrove distribution. The root and rhizomes of mangrove will modify the sediment become muddier, changing the species composition of crab community. In Taiwan, Mangrove species, Kandelia obovate, was introduced to Tamsui and has become the dominant plant species. These years, it expands continuously. The effect of mangrove expansion on crab species distribution in Taiwan is still unclear. We assessed how the crab species and mangrove distribution changes between 2001 and 2014. Mangrove coverage both expanded and reduced in our transects. In the reducing mangrove, the density and distribution area of crab species Macrophthalmus banzai decrease, with Uca borealis appear. In the expanding mangrove, only M. banzai was found inside the mangrove. Our results show that mangrove expansion may lead to lower crab species richness with only generalist crab species found in the mangrove. Recently, due to global warming, mangrove may have the access to expand to the colder area. This research provides an example of how the dynamics of mangrove coverage affect crab communities on tidal mudflats.