In order to improve the stability and climbing performance of crawler mini-vehicle working on rugged roads and slope lands, a deformable crawler travelling unit was designed by using a three-linkages mechanism, driving by a hydraulic cylinder fixed on walking frame to achieve the horizontal position of walking frame within a certain range by deformation of the crawler. The force of links at equilibrium position and motion ability of mechanism were analyzed by the degree of freedom calculating and the mathematical modeling of the unit. The relationship between the angles of deformation and the position of the components in the mechanism was determined. The vertical obstacle surmounting and climbing performances of the unit operating in deformable and un-deformable mode were compared. It is shown that the unit working on deformable mode can self-adaptive climbing the slope below 28°, and improved 3° climbing ability while decreasing the ability of vertical obstacle surmounting.