During tube hydroforming process, fracture often occurs on the aluminum alloy tubes due to their low plasticity and poor formability. Therefore, the topic about formability improvement of hard-to-form tubes is of great importance and attraction in the field of tube forming. In this paper, the 5052 aluminum alloy tube was covered with a 304 stainless steel tube with good plasticity as a supporting in order to enhance its formability. Then, the double-layered tubes were hydrobulged freely under the internal pressure simultaneously. It is shown that the bulging ability of inner 5052 aluminum alloy tube can be enhanced dramatically by the outer supporting tube. The ultimate expansion ratio can be improved from 11.57 to 30.86 %, corresponding to an approximate increasing rate of 167 %. The outer supporting tube can make the 5052 aluminum alloy tube achieve a greater thickness reduction before fracture occurrence. Moreover, the outer supporting tube has also an influence on the fracture behavior of 5052 aluminum alloy tube. The microscopic dimples on the 5052 aluminum alloy tube fracture surface have become bigger and more flat, corresponding to a reduction of the number and area of microscopic dimples. The equivalent fracture strain of 5052 aluminum alloy tube is deceased when the outer supporting tube was used. In brief, the formability of 5052 aluminum alloy tubes can be enhanced dramatically by the outer supporting tube, and the deformation uniformity can also be improved.