The three dimensional magnetic field and field parameters H0

H2

H4 produced by deflection yoke with several ferromagnetic loops are calculated by using the boundary element method（BEM）.These ferromagnetic loops may be of rotational symmetry（such as Toroid core）or multipolar symmetry（such as CROSS）.Fourier harmonic analysis method is used to transform the three dimensional integral to two dimensional integral.It can save the memory units and the calculating time of the computer.The singularities in calculating the diagonal elements of the matrix are manoeuvred to obtain accurate result.As an example

a parallel saddle coil with several ferromagnetic loops has been calculated in this paper.The theoretical calculated results of the field parameters coincide very well with the measured results by our new designed deflection magnetic field measuring instrument.