A kind of automatic measuring instrument for 3-D river bed model topography is introduced.The instrument adopts the theory of ultrasonic echo for quickly scanning river bed topography under water and employs the principle of resistance for measuring river bed topography upon water and shallow beach topography, which increases the measuring speed effectively and decreases the disturbing influence on river bed. Because of using moving framework facility and proper computer controlling method, the testing bridge can move freely lengthways. The instrument can automatically accomplish the measurement of 3-D river bed topography,the display of three dimension topography and the drawing of contour map.