In the era of big data, the scale of data centers has been increasing year by year. A data center has high energy consumption, high elasticity and interconnected load flexibility. However, current power system operational reliability evaluation research ignores the load flexibility of the data center, resulting in inaccurate evaluation results and an inability to reflect the energy efficiency status of the data center. This paper first proposes a data center load flexibility model based on the interconnection between data centers and the off-site transfer characteristics of computing power loads. Second, a power system operational reliability model including data center energy efficiency and operational reliability contributions is established. Further, considering random factors such as electrical load, data center computing power load, and wind and solar power, a power system operational reliability evaluation model considering data center load flexibility is established, and a corresponding evaluation method is proposed. Finally, through simulations on the IEEE RTS and a practical power system, the results show that the load flexibility of the data center not only contributes to the consumption of renewable energy and the safe and reliable operation of the power system, but also helps the data center to save energy and reduce consumption.