The Z-pinch experiments with a reflux hood structure, exp90 or exp60, were carried out in an intense pulsed power device FP-2 and simulated and analyzed by the two-dimensional magnetically driven simulation code (MDSC2). The numerical simulations show that the measured current/loop current is not a load current of liner in the Z-pinch experiment with the reflux hood structure. There is a structure coefficient between the measured current/loop current and the load current. A new formula for the relation between the boundary magnetic field and the loop current is presented. The new boundary magnetic field formula with the structure coefficient and the MDSC2 code can correctly simulate the Z-pinch experiments with a reflux hood structure. The simulated inner diameter velocity is consistent with the measured one by Velocity Interferometry System for Any Reflector (VISAR). The structure coefficients is a constant, which is determined only by the initial structure of Z-pinch with reflux hood structure. In the Z-pinch experiments with the reflux hood structure, the structure coefficients for the 90 mm inner diameter liner and the 60 mm one are 0.87 and 0.90 respectively. When the initial thickness of liner, insulation material and other conditions are the same, the larger the inner diameter of liner, the smaller the structure coefficient of Z-pinch with reflux hood structure.