A pressure measurement method for in-service vessel based on magneto-mechanical effect is proposed. Non-intrusive and non-contact pressure measurement is realized by using the stable correspondence between magnetic signals outside the vessel and the stress on the vessel wall. The relationship between magnetic permeability and stress of steel in weak geomagnetic field is analyzed by using J-A coupling model, and the feasibility of using external magnetic field of vessel to measure internal pressure is theoretically proved. A multi-channel synchronous magnetic signal acquisition system is designed, which can simultaneously acquire the magnetic signals of three components of the multi-sensor. Experiments are carried out to verify the performance and advantages of this method. The results show that in the range of 0-3 MPa, there is a good linear relationship between the magnetic field near the surface and the internal pressure of the vessel. The sensitivity to the pressure change of the magnetic field at different parts of the vessel surface is different, so multi-point deployment and calibration optimization are required. For low carbon steel pressure vessel with an outer diameter of 275 mm and a wall thickness of 7.5 mm, the sensitivity of magnetic measurement can reach up to 131.4 mGs/MPa. Via axis-symmetrically arranging a pair of sensors and adding their output up, the influence of rotating vessel on measurement accuracy can be significantly weakened.