Bitcoin is a crypto currency introduced by Satoshi Nakamoto in 2008. It has the features of decentralization, cross-border and fixed total amount and has become one of the most widely used crypto currencies. Due to some initial limitations set by the inventor and the following developers, the transaction throughput of the Bitcoin network is much limited. Recently, the transaction throughput has been close to the maximum limit, and the corresponding transaction confirmation time has been greatly increased. Not only this affects user experiences of Bitcoin and limits its usage, but also this puts forward higher requirements for Bitcoin protocol design. Focusing on the challenges of transaction processing performance, this paper aims to promote blockchain capacity and takes a deep research on Bitcoin protocol. Firstly, we do a research on the current network status of Bitcoin, and analyze the transaction delay according to Bitcoin transaction data. Secondly, we analyze the feasibility and effectiveness of on-chain scaling proposals. Thirdly, we analyze mechanics and effects of off-chain scaling proposals. Finally, we analyze the advantages and disadvantages of on-chain/off-chain scaling proposals, and propose a scaling roadmap which meets the community requirements. The recent progress on the Bitcoin scaling shows the correctness of our proposals.