Liquid supplementing system and liquid supplementing method for pressure-controlled drilling

The invention provides a fluid infusion system for pressure-controlled drilling. Comprising a first channel connected between a drilling pump and a vertical pipe, a second channel connected between the drilling pump and a well annulus outlet formed between a drilling machine and a shaft, a third channel connected to the second channel in parallel, and a fourth channel connected between the first channel and the third channel. The fifth channel is connected between the third channel and a slurry circulating treatment system in a pressure-controlled drilling loop; wherein in the first state, the liquid supplementing system is controlled to open the first channel and close other channels to conduct the pressure-controlled drilling loop, so that vertical pipe circulation operation is carried out, and in the second state, the liquid supplementing system is controlled to guide flow to the second channel, so that slurry flow guide operation is carried out. The first state and the second state can be switched. The in