A traditional model of energetic ecological footprint was modified through adding artificial input indexes, unifying energy density and introducing energy-releasing/standard-coal coefficient, based on the assessment of the traditional model. Then, the sustainable development level of Nantong City, Jiangsu Province from 1996 to 2010 was measured with the modified model. Results show that: (1) The ecological footprint of Nantong City increased from 0.65 to 0.90 hm² per capita from 1996 to 2010, caused by the rapid energy consumption. (2) The ecological carrying capacity of Nantong City increased from 0.51 to 0.81 hm² per capita during the period, supported by the growth of agricultural and hydropower production, with a large potential of labor and capital input. (3) Nantong City has been in a state of ecological dificit, and the sustainability evaluation index was always larger than 0.5 from 1996 to 2010, which means that Nantong City was in an unsustainable state. In order to narrow the gap between ecological footprint and ecological carrying capacity, some measures can be put forward: guiding major industries and enterprises to save energy and exploiting new energy actively; increasing the investment in agricultural technology to improve agricultural production; and paying attention to investment in human resources and promoting technology and capital intensified economy.