BmP02 is a 28-amino acid residue peptide purified from the venom of the Chinese scorpion Buthus martensi Karsch, which had been demonstrated to be a weak blocker of apamin-sensitive calcium-activated potassium channels. Two-dimensional NMR spectroscopy techniques were used to determine the solution structure of BmP02. The results show that BmP02 formed a alpha/beta scorpion fold, the typical three-dimensional structure adopted by most short chain scorpion toxins whose structures have been determined. However, in BmP02 this alpha/beta fold was largely distorted. The alpha-helix was shortened to only one turn, and the loop connecting the helix to the first beta-strand exhibited conformational heterogeneity. The instability of BmP02 could be attributed to a proline at position 17, which is usually a glycine. Because the residue at this position makes intense contact with the alpha-helix, it was supposed that the bulky side chain of proline had pushed the helix away from the beta-sheet. This had a significant influence on the structure and function of BmP02. The alpha-helix rotated by about 40 degrees to avoid Pro17 while forming two disulfides with the second beta-strand. The rotation further caused both ends of the helix to be unwound due to covalent restrictions. According to its structure, BmP02 was supposed to interact with its target via the side chains of Lys11 and Lys13.