The numerical simulation of multipactor in the dielectric surface breakdown in high direct current field is realized by using the particle-in-cell method. And the influence of the strength of the high direct current field, smoothness of the dielectric surface and secondary electron yield coefficient on the multipactor are researched through the simulation of multipactor. Finally, the influence of the tilting of high direct current field and external magnetic field on the multipactor are also investigated. The results show that selecting of the dielectric with low secondary electron yield coefficient and tilting of high direct current field can reduce the degree of multipactor, and for the external magnetic field the degree of multipactor decreases effectively only when the external magnetic field exceeds a certain value.