In order to estimate the losses and temperature rise at the end of electrical machines, it gets more and more important to make detailed calculation on end region magnetic field of large turbogenerators. The magnetic field distribution in end region of a large generator is complicated. It is necessary to adopt 3D FEM. To investigate into the clamping bolts failure of a generator, a computation on 210MW turbogenerator is made, where load magnetic field in end region is analyzed directly with software FLUX in two scalar potential in this paper. The field domain not only consists of pressing plate, clamping bolts and stator and rotor windings, but also parts of stator and rotor core in the end. The operational conditions for calculation mainly include no load, rated load with stator end winding at 30° and 31.5° conical angle. Comparison of calculated results and measured results are made in a turbogenerator of Qinghe power station.