The invention discloses a method for measuring altitude difference and a digitalizer. The method is as follows that an instrument measures four inclination angle values and a point-point horizontal included angle; the point-point topography altitude difference data can be obtained according to the triangle principle; and then the horizontal distance and the gradient data of the point-point topography can be obtained by using the cosine theorem and gradient mathematic expression. The digitalizer comprises an optical laying component, a horizontal rotating disc, a display screen, an electric-control function unit, an external control unit and an A-frame component. The optical laying component comprises a telescope and a tilt angle sensor arranged on the telescope. The horizontal rotating disc is arranged on the A-frame component, is provided with the display screen and the external control component, and is internally provided with an angular displacement sensor arranged on the rotation axis center of the horizontal rotating disc and an electric-control function unit. The method and the device can fast accurately measure the point-point altitude difference, the horizontal distance and gradient in time.