In the vertical target density measuring, in order to build larger measuring target area (such as 10 m×10 m) and realize higher precision of the measuring system, an optical detector was put into the acoustic array so that a new measuring model was established. The measuring model has the features of both larger measuring target area of the acoustic array and higher precision of the optical detection. The measurement formula was deduced, and the measurement error was analyzed and simulated. The theory measurement precision of 5 mm of x coordinate error and 15 mm of y coordinate was obtained.