When the president. Negotiate a treaty with China and will lower the taxes on imports from China which role does the president fill

The president is filling the role of head of state.

Explanation:

The head of state is the leader of the nation. The person who embodies the state in international affairs, and represents it when establishing a relationship with another country.

In the United States, the head of state, and the head of government, is the President, this is why the United States has a presidential system, while for example, the United Kingdom has a parliamentary system, where the head of state is the Queen, while the head of government is the prime minister.

The President of the United States represents the country in the international state, and is allowed to negotiate and sign treaties with other countries like China, with the advise and consent of the Congress.