At present, the research on bamboo mainly focuses on large-diameter bamboos such as moso bamboo, and the research and application of square bamboo and small-diameter bamboos are rare. Based on the test method of physical and mechanical properties of bamboo, the mechanical tests were carried out on 111 Zhaotong square bamboo flake samples born in 3~5 years, and the tensile and compressive strength, elastic modulus and other parameters were determined. Based on the experimental data, the intensity probability distribution was determined and its distribution characteristics were verified. Combined with the research results of timber structure and the reliability limit state design method, the strength standard value and design value of square bamboo were obtained, and its feasibility was verified by comparison with moso bamboo, larch and other materials. The reliability index was corrected for the design value of the compressive strength of the grain that did not meet the requirements of the code. The result enriches the database of mechanical properties of small-diameter bamboo, and also opens up a new direction for the further research and application of bamboo.