(1): powering on a coil, and then using the coil to excite a specimen under test (hereinafter referred to as specimen), then an induced eddy current will be generated in the specimen, meanwhile, in the process of heating, using an infrared camera to acquire a thermogram sequence of the specimen, which is denoted by thermogram sequence S, where each frame of image of thermogram sequence S is m*n;

(2): for each location of thermogram sequence S, putting pixels there according to time to fit a temperature-time curve, then choosing a slope of a second point of the temperature-time curve as a rate of change of temperature with respect to time, which is denoted by

(3): creating a current matrix by:

3.1) calculating a current amplitude for each pixel of the thermal reference image according to the following heat conduction equation:

where J_ij is a current amplitude of row i and column j, i=1, 2, . . . , m, j=1, 2, . . . , n, σ* is a conductivity of the specimen, ρ is a density of the specimen, C is a specific heat capacity of the specimen;

3.2): creating a current matrix J:

(4): creating a magnetic potential matrix by:

4.1): calculating a magnetic potential for each pixel of the thermal reference image according to the following equation:

where I is a current in the coil, μ₀ is a permeability of air, C is a closed path along the coil, dl is a vector length element along the coil, R_ij is a distance from the coil to the pixel of row i and column j of the specimen;

4.2): creating a magnetic potential matrix A:

(5): calculating the electric potential and conductivity for each pixel of the thermal reference image according to the electrical impedance tomography by:

5.1): inputting initial conductivity matrix σ¹, angular frequency ω and iteration termination criteria ε, and initializing iteration number k=1;

5.2): for iteration k, calculating an electric potential matrix Û^k by traversing all pixels of the thermal reference image, where the electric potential U^k_i,j of row i and column j satisfies the following equation:

where parameters a_i,j, b_i,j, c_i,j, d_i,j, e_i,j, f_i,j satisfy the following equation:

then creating the following matrix form:

where U^k is a electric potential vector, which is created by choosing electric potentials U^k_i,j from left to right and top to bottom, and putting them from top to bottom:

where parameter matrix G is composed of parameters a_i,j, b_i,j, c_i,j, d_i,j, e_i,j:

where vector C is composed of parameter f_i,j:

then calculating electric potential vector U^k according to the following equation:

then choosing the electric potentials of electric potential vector U^k from top to bottom, and putting them from left to right and top to bottom to create electric potential matrix Û^k:

5.3): calculating electric field intensity matrix E^k of the k^th iteration, where electric field intensity E^k_i,j of row i and column j satisfies the following equation:

where electric field intensity matrix E^k is:

5.4): calculating the conductivity matrix σ^k+1 after the k^th iteration, where conductivity matrix σ^k+1 is:

where conductivity σ^k+1_ij of row i and column j is:

5.5): judging whether the infinite norm ∥σ_k+1−σ_k∥_∞ is less than iteration termination criteria ε, if not, then adding 1 to iteration number k and returning to step 5.2) to continue iterating, otherwise, going to step (6);

(6): outputting conductivity matrix σ^k+1, and taking it as a reconstructed image, then taking the low conductivity area of the reconstructed image as the defect profile.