Abstract: To enable a sustainability assessment of integrated energy systems, an emergy indicator has been introduced to describe the sustainable development level of system. An integrated energy system based on photovoltaic-gas-storage complementarity was constructed, with the maximization of the emergy sustainability index (ESI) in emergy analysis methods set as the objective function to optimize the equipment capacity allocation. Based on two operation strategies, namely following the thermal load (FTL) and following the electric load (FEL), and considering material consumption and pollutant emission costs over the full life cycle of system in addition to traditional emergy analysis methods, a genetic algorithm was applied to identify the optimal capacity configuration of the integrated energy system, and the emergy performance under the two strategies was analyzed and compared. Results reveal that the ESI of the integrated energy system is 2.85 under the FTL strategy and 2.06 under the FEL strategy. The ESI under the FTL strategy is 38.35% higher than that under the FEL strategy, indicating better sustainability.