The preset type belt automatic self-aligning idlers are composed by vertical roller mechanism, roller mechanism and link mechanism, when the belt running in deviation, the belt edges will push the vertical roller moving towards the deviation direction of the belt , the vertical rod in the vertical roller will move at the same time, the vertical rod will drive the roller mechanism to deflect simultaneously, the linear motion direction of the deflection roller and the belt running direction will form an angle, the resulting lateral friction will drive the belt to move in the middle.