Abstract: In order to study the mechanical properties of the long hollow box with stiffened bar, this study measured the bearing capacity under different load-mass ratio of 147 test pieces of various length and cross-section, which are made of bamboo skin and strips using distinct methods. The test results show that the failure of the bar with length within 250 \sim 350~mm is between compression failure and instability failure, and the bearing capacity and load-mass ratio for double-layer bars are better than those of single-layer bars. It is necessary to repair/strengthen the defects of bamboo skin and the edges of the bar during the preparation progress. This study is part of a college competition of structural design of bridges, aiming to cultivate students innovation in combining mechanical theory and engineering.