Abstract: The originally designed restrained mandrels used in Accu-Roll tube mill in Yantai Lubao Steel Tube Corp., Ltd. were solid single ones with less cooling time and high and non-uniform temperature on their surface when used in operation, resulting in bad adhered lubricant on their surface, fast wearing and shortened working life of them. After the mandrels are modified to the ones with combined inside and outside water-cooling, the cooling effect greatly improved, the working life elongated and the surface quality of pipes improved.