A convertible axle structure includes a first shaft, a second shaft parallel to the first shaft, and a switch member disposed between the first shaft and the second shaft. The outer wall of the first shaft has a slide groove composed of a front radial slide groove portion, an axial slide groove portion, and a rear radial slide groove portion. The outer wall of the second shaft has a guide groove composed of a radial guide groove portion, a front inclined guide groove portion, and a rear inclined guide groove portion. The switch member has a first protrusion slidable in the slide groove and a symmetrical second protrusion slidable in the guide groove to guide the first shaft and the second shaft to carry out an asynchronous convertible turning.