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To measure the amplitude and phase of the non-axisymmetric radial magnetic field generated by the locked mode, 12 saddle loop sensors are newly developed on the J-TEXT Tokamak. The saddle loop is made of flexible printed circuit (FPC) to adapt the complex installment environment and ensure the installment accuracy. In the experiment, the saddle loop measures the radial magnetic field of locked mode and the axisymmetric equilibrium magnetic fields as well as that of the corresponding eddy current. Precise compensation for the fluxes induced by the horizontal and vertical field coils is realized by utilizing the lumped eddy current circuits based on an analytical model. By using this set of saddle loop sensors, the amplitude and phase of the m/n = 2/1 locked mode are clearly measured for the case of error field locking with slow rotation and the penetration of resonant magnetic perturbations (RMPs) respectively. Here, m and n are the poloidal and toroidal mode number.