In this paper, the magnetic field distribution around a coil is worked out by solving the Maxwell equations for both the cases before and after inserting a "concentrating body" into the winding coil. The solution shows that, if we keep the current in the winding coil constant before and after then inserting, then the magnetic field in the hollow region will not change; but if we keep the supplied energy unchanged before and after the inserting (i.e. C, V fixed), then the ratio of magnitude of the field inthe two cases Will be B/B0∝(R0)1/2. The latter effect results, however, completely from redistribution of the field. The total flux in the hollow region is reduced after the inserting, the fluix is not costant, there is not any flux concentrating effect. Hence the expectation of using such an effect to produce pulsed intense magnetic field is misleading in principle.We have also analysed why it is wrong to regard the flux concentrator as a pulse transformer. The theoretical calculation further shows that the increasing of magnetic field obtained by using devices with truncated cone shaped endings is due to the effect of construction.