In order to calculate theoretically the coil impedance, the separation of variables method and the Cheng matrix method are used to establish the analytical model by applying the magnetic insulation condition in eddy current tube detection with encircling probes inside and outside respectively. In the established model, there exists a definite integral related to modified Bessel function. Gaussian integral algorithm is proposed to accurately perform the numerical calculation. Simulations are carried out on the thinning of a copper tube wall thickness using the presented model, traditional Dodd model and finite element model. Their results are in good agreement with each other, demonstrating that the derived analytical model is correct. Compared with the Dodd and finite element models, the improved model has some advantages such as good efficiency, easy adjustment of accuracy and so on.