The invention provides an online gas pressure-tapping method and an online gas pressure-tapping device. The method comprises the steps of (1) mounting the pressure-tapping device, (2) resetting the device, and (3) carrying out gas pressure-tapping. The device comprises a main ball valve, a pressure transmitter, a tee joint, an auxiliary ball valve and a U-shaped pipeline; one end of the main ball valve is communicated with a gas pipeline by use of a pressure-tapping pipeline, while the other end of the main ball valve is connected with a lower opening of a three-way joint by use of a pipeline; the auxiliary ball valve is connected with an upper opening of the three-way joint by use of a pipeline; one end of the U-shaped pipeline is connected with a right opening of the three-way joint, while the other end of the U-shaped pipeline is connected with the pressure transmitter. The online gas pressure-tapping method and the online gas pressure-tapping device have the advantages that the traditional mounting manner is broken through, the working efficiency is improved, necessary isolation measure is carried out, the pressure-tapping pipeline is convenient to dredge, and the safety and accuracy are greatly improved.