The invention discloses a two-dimensional precise positioning auxiliary device of a portable sonar depth finger. The device comprises a vertical device, a horizontal device and a GPS positioning device; the horizontal device is detachably fixed on a fixed facility at the shore of a to-be-measured riverway, a connecting end part of the vertical device is arranged on the horizontal device in a sliding connection way, and capable of moving back and forth along a length direction of the horizontal device; the vertical device is a length-adjustable telescopic structure, and a fixing device for fixing the depth finder in the sliding connection way is arranged at a free end part thereof; the portable sonar depth finder disclosed by the invention can move up and down at a vertical direction at a fixed point, thereby realizing the sludge measurement of the riverway; a scale identifier is arranged on the vertical device so as to measure the distance to riverway edge from the portable sonar depthfinder; and the GPS device is arranged at the same location, right above or below of the fixed point of the portable sonar depth finger by the free end part of the telescopic structure, and is used for measuring longitude and latitude of the location located by the depth finger sludge measurement point.