The invention discloses a linear cutting machining device and method for an internal spline of a step. The device comprises a linear cutting machine tool, a molybdenum wire direction changing device matched with the linear cutting machine tool, an insulation cushion block and an index plate provided with an inner hole. The molybdenum wire direction changing device enables a molybdenum wire to pass through a stepped hole of a workpiece and then be deflected towards the inner wall of the workpiece. The linear cutting machining device is simple in structure, low in manufacturing difficulty, easy to operate in the using process, safe and reliable. By means of the device and the method, a spline key groove can be completely machined, a step hole wall is not damaged, the quality requirement of products can be met, and the manufacturing cost of the products can be reduced. The machining problem of the step internal spline which is high in material rigidity is solved.