The invention discloses a method for preparation of composite insulator solid mandril, including: leading-out setting quantity of epoxy glass fiber to a dipping tank for dipping, the epoxy glass fiber after dipping entering into a mould pressing machine, arranging in a preheat die coated with release agent; the mould pressing machine further heating the die, simultaneously applying the identical pressure to the top and bottom of the die, after heating the die at 80 DEG C - 180 DEG C for 4 - 8 hours, maintaining pressure at 180 DEG C - 120 DEG C for 1 to 3 hours to complete the mandril by one time molding; after the one time molding of the mandril, stating the mould pressing machine, separating the dies, drawing the molding epoxy glass fiber from the die, and performing second curing by microwave heating. To ensure complete curing in the internal of the mandrel, intermittent pultrusion technology is combined with the flat-plate heating molding technology, to realize the production of large-diameter epoxy glass fiber composite insulator mandril.