Abstract: This paper focuses on the optimal operation of the park-level integrated energy system (PIES), and the current status of research is analyzed from three aspects: model, algorithm, and mechanism. Firstly, the research status of the PIES is expounded from the modeling aspects. The basic optimization model, the uncertainty model, and the game theory-based model are discussed, respectively. Secondly, algorithms employed in the optimal operation of the PIES are summarized, including the mathematical programming method, the heuristic method, and the reinforcement learning method. In addition, the effects of the key mechanisms on the optimal operation of the PIES are discussed. Influences brought by the mechanisms of the integrated demand response, emission trading market, green certificate market, and integrated energy market are outlined and analyzed. Finally, this paper puts forward prospects in the development trends of the optimal operation of the PIES. The opportunities and the challenges of the PIES are discussed from the decarbonization, marketization, energy sharing, decentralization, and digitalization aspects, respectively.