The invention discloses a process for assisting in foaming of cross-linked polyolefin as well as an ultrasonic treatment device for the process. The ultrasonic treatment device is configured in a common foaming furnace to ultrasonically treat a polyolefin foaming material which is just foamed but still in a molten state. Lots of small holes are formed in a polyolefin melt by high energy and strong cavitation effect of ultrasonic vibration, so that the free energy barrier for bubble nucleation is reduced, bubble formation is facilitated, the total quantity of bubbles is increased and the average diameter of bubbles is reduced. In addition, large bubbles can be broken into multiple small bubbles by strong ultrasonic vibration, and the small bubbles are re-surrounded by the polyolefin melt, so that the bubbles in the melt are more uniform, and the polyolefin material with densely and uniformly distributed air holes can be obtained.