In laser plasma beat wave accelerator，the strong magnetic field higher than 1500 Guass, which can confine the behavior of plasma, is needed .In this paper, the distrib ution of magnetic field on plasma generator is obtained using OPERA3D procedure. The simulation agrees with the experimental data well. While adjusting some parameters，some factors, which cause the actual magnetic f ield lower than the designed one, are found . Based on this simulation, modification has been given．