Over the past several decades, high performance computing (HPC) in China has undergone tremendous growth under the continuous support of national research programs. The development of exascale computers is the current goal set by the National Key R&D Project on HPC. Starting with a brief historical review of China's HPC development, this article analyzes the major challenges encountered in developing exascale computers. Thereafter, some important issues in realizing exascale computing are discussed, including architecture, processor, interconnect, parallel system software, parallel programming, algorithm, and resilience.