We studied the safety threshold of an ecosystem in the Er-Ka Nature Reserve in the Inner Mongolia Autonomous Region of China, a typical floodplain wetland supplied by the Hai-La-Er River. Because information on the Er-Ka Nature Reserve is scarce owing to a lack of research done in the area, the safety threshold of the ecosystem is difficult to ascertain through traditional ecological methods. However, the floodplain wetland was closely associated with the river hydrology, and information on the river hydrology is abundant. It was therefore possible to study the safety threshold of the ecosystem by analyzing the relationship between the Er-Ka Nature Reserve and the Hai-La-Er River, using both ecological and hydrologic methods. The flow of the Hai-La-Er River was measured by obtaining 250 flow values every year from May to September from 1961 to 2010. From the data, three periods were selected to represent the low flow, middle flow, and flood periods, with runoffs of 58.72 m³/s (P=74.6%), 119.26 m³/s (P=44.4%), and 190.35m³/s (P=74.6%), respectively. The wetland status in the Er-Ka Nature Reserve during these three periods was studied using satellite images. Results demonstrate that the wetland area in the Er-Ka Nature Reserve was 2797.5 hm² at the end of the low flow period. The majority of the area comprises herbaceous swampland (50.31%) and salinization swampland (24.44%). At the end of the middle flow period, the wetland area was 3488.3 hm², of which the temporary river wetland, herbaceous swampland, and salinization swampland made up 28.40%, 25.14%, and 28.31%, respectively. At the end of the flood period, the wetland area was 4522.0 hm², 52.3% of which was temporary river wetland. Lake wetlands, the core areas of the Er-Ka Nature Reserve, were only 47.3 hm² at the end of the low flow period, far less than the area they covered during the middle flow period (259.2 hm²), and in the flood period (2365.0 hm²). Additionally, the degree of fragmentation of the lake wetland was 1.37 in the low flow period, which is much higher than that in the middle flow period (0.57) and in the flood period (0.62). These findings indicate that the wetland status of the Er-Ka Nature Reserve was unstable at the end of the low flow period. Thus, the status of the wetland at the end of the low flow period could be regarded to be approximately the safety threshold of the ecosystem, at which point the ratio of wetland area to the Er-Ka Nature Reserve area was above 43.03%, while the ratios at the end of the middle flow (53.66%) and flood (69.56%) periods may be acceptable wetland to reserve area ratios. This study can provide support for the management and protection of the Er-Ka Nature Reserve, and can provide a reference for future floodplain studies.