The emergy indices for the evaluation of system s sustainable development ability were studied. Results indicated that the emergyindices are simplified and merged, and a new emergy index for sustainable development(EISD) is deduced. Employing EISD, two cases areconducted. The first one is to compare three different dike-pond agro-ecological engineering modes, which are: melon-melon-cabbage-fourdomestic fishes (mode Ⅰ ), melon-melon-cabbage-pig-four domestic fishes (mode Ⅱ ) and melon-melon-cabbage-pig-four domestic fishescombined with Siniperca chuatsi B. ( mode Ⅲ ). The result is that the EISD of mode Ⅰ is 0.53. Mode Ⅱ s EISD is 5.26 times of mode Ⅰ ,and mode Ⅲ s EISD is 6.83 times of mode Ⅰ . The second one is to evaluate the development of Zhongshan City, Pearl Delta, during 1996to 2000. The result indicated that the EISD of Zhongshan had appreciably declined from 1996 to 1998, and quickly improved from 1998 to2000, partly because of its environment protection and product construction. Both of the two cases studies showed that EISD can assessment thesustainable development ability more roundly, with the consideration of environmental impact and social-economic effect at the same time.