A crossbow assembly includes a body having an insertion hole and a retention member protrudes from the insertion hole. A barrel has a first end inserted into the insertion hole and a second end of the barrel extends beyond the insertion hole. A recess is defined in the top of the second end of the barrel. An upper portion is connected to the top of the barrel. A lower portion is formed at the underside of the barrel. The thickness of the upper portion is smaller than or equal to a half of that of the lower portion. A bow unit is connected to the second end of the upper portion and includes a riser, two limbs and two cam units. The upper portion is connected to two plates extending from a slot in the riser by two bolts. The crossbow assembly has a stable weight center.