A downhole blowout preventer has a blowout preventer tubing string containing a circulation unit, a packer unit and a pressure-building unit connected in sequence from top to bottom. The pressure-building unit is configured to be activated in response to a command, so as to build up pressure in a space inside the blowout preventer tubing string. The packer unit is configured to be activated when a pressure in said space reaches a first predetermined value, so as to seal an annulus between said blowout preventer tubing string and an external casing or borehole. The circulation unit is configured to be activated when the pressure in said space reaches a second predetermined value, thereby creating a circulation path between an interior and an exterior of the blowout preventer tubing string. The first predetermined value is less than the second predetermined value.