Bitcoin is a decentralized peer-to-peer electronic cash system, and it realizes the circulation and distribution of cryptocurrency via miners contributing their computational power to solve a cryptographic puzzle. With more and more people participating in Bitcoin mining, the continuous growing of the computational power might greatly reduce the time for finding a new valid block. To keep the stability of the Bitcoin system, the difficulty of the Bitcoin network will be adjusted every 2016 blocks. This paper aims to study the principles of the difficulty adjustment of the whole Bitcoin blockchain network in Bitcoin mining. We first describe the computing process of the target and thus the difficulty of a block, and then study its influence factors and their effect on the difficulty. After that, we study the adjustment of the difficulty, and propose a framework for the adjustment of the difficulty that can be easily used in practice. Our work can help researchers better understand the principles of Bitcoin mining.