The invention relates to a graphene composite absorbent and a preparation method thereof. The method comprises the following steps of expanded graphite preparation: processing graphite oxide by microwaves to obtain the expanded graphite; graphene micro sheet preparation: mixing the expanded graphite, 3-amino propyl triethoxysilane and liquid alcohol, and performing ultrasonic treatment on the mixture to obtain the graphene micro sheets; ball milling: mixing the graphene micro sheets, carbonyl iron powder and grinding aids; performing ball milling; cooling products obtained after the ball milling; then, performing solid-liquid separation; collecting solid products; performing cleaning and vacuum drying on the solid products to obtain the graphene composite absorbent. The composite absorbenthas the characteristics that the weight is light; the broadband wave absorption is realized, and the like. Compared with a conventional graphene absorbent, the graphene composite absorbent has the obvious advantages in aspects of electric performance, magnetic performance and the like, so that the specific frequency band electromagnetic wave absorption function is realized, so that the absorbentcan be used for making coating type wave absorption materials and structure type wave absorption materials so as to improve the target hiding features.