Mainland Chinese citizens can buy Bitcoin in the United States through various methods. Here are some ways they can do so:

1. Use a cryptocurrency exchange: Mainland Chinese citizens can open an account on a cryptocurrency exchange that operates in the United States. They can then deposit funds into their account using various payment methods, such as bank transfers or credit/debit cards, and use those funds to buy Bitcoin.

2. Use a peer-to-peer platform: Mainland Chinese citizens can also buy Bitcoin on peer-to-peer platforms that connect buyers and sellers directly. They can find sellers in the United States who accept payment in Chinese yuan or other currencies, and then arrange a transaction to buy Bitcoin.

3. Use a Bitcoin ATM: Mainland Chinese citizens who are in the United States can also buy Bitcoin using a Bitcoin ATM. These machines allow users to purchase Bitcoin with cash or credit/debit cards, and the purchased Bitcoin is transferred to their digital wallet.

4. Use a Bitcoin brokerage service: Mainland Chinese citizens can also use a Bitcoin brokerage service that allows them to buy Bitcoin through a broker. They can provide their payment details and the broker will handle the transaction on their behalf.

5. Use a Bitcoin investment fund: Mainland Chinese citizens can invest in Bitcoin through a Bitcoin investment fund that is based in the United States. These funds allow investors to buy shares in a fund that holds Bitcoin, providing exposure to the cryptocurrency without directly owning it.

It is important for mainland Chinese citizens to be aware of the legal and regulatory considerations when buying Bitcoin in the United States. They should ensure that they are compliant with any relevant regulations and that they are using a reputable and secure platform for their transactions. Additionally, they should consider the tax implications of buying and holding Bitcoin in the United States.