The invention provides a step adjustable belt wheel and a belt wheel tensioning device. The step adjustable belt wheel comprises a right-side belt pulley and a left-side belt pulley; the sides, which are connected with each other, of the right-side belt pulley and the left-side belt pulley are separately provided with a slope, and the slopes form a belt groove jointly; the right-side belt pulley and the left-side belt pulley are connected through a connector; and the size of a clearance between the right-side belt pulley and the left-side belt pulley can be adjusted through the connector. The step adjustable belt wheel and the belt wheel tensioning device provided by the invention are simple in structure, small in adjustment space and wide in adjustment range, and front and back cross tensioning is realized; a wrap angle of a driving wheel is not reduced, and the belt wheel can be tensioned; parts do not need to be changed, and tensioning is realized through adjustment.