Distributed energy system (DES) has become favorite topic for energy saving, but exposed to a range of technical constrains on the system structure, operation strategy and optimization method with gradually accelerating the penetration. Thus, it is significant to study how to make some improvements from these three aspects. This paper proposes a new DES integrating cogeneration, photovoltaic and ground source heat pump (GSHP), which is modeled from energy balances and flows. Considering the high-dimension of the DES without any capacity assumptions, a multi-objective optimization method is built to analyze the annual energy saving ratio (AESR), cost saving ratio (ACSR) and emission reduction ratio (AERR), based on the differential evolution and particle swarm optimization hybrid algorithm and analytic hierarchy process. Then taking a large office in Beijing as the baseline building, a seasonal operation strategy coupled with seasonal adjustment of the cooling/heating share provided by GSHP is developed. The sensitive analysis of the photovoltaic capacity, energy price and objective weights is also performed to show the variation rules of the benefits under different optimization conditions. Results show that the integrated energy system under seasonal operation strategy outperforms the separate system with the AESR, ACSR, AERR of 17.85%, 55.50% and 34.22%. (C) 2017 Elsevier Ltd. All rights reserved.