The invention discloses a graphical estimation method for an optimal detection frequency under array size constraint. The method comprises the following steps: acquiring an initial parameter; selecting an operating frequency set and determining an absorption coefficient set corresponding to the operating frequency set; determining the expression of a constant threshold C and a frequency distance correlation function yi at the operating frequency f: performing graphics processing, and orderly drawing the frequency distance correlation function Yi in a distance range r and drawing a straight line of the constant threshold C; determining an intersection Pi (ri, C) of the straight line of the constant threshold C and the frequency distance correlation function Yi; obtaining the maximum value rp in set [r1, r2, to ri, to rw] by solving, and determining an operating frequency fp as the optimal operating frequency of sonar. By drawing a curve and a constant threshold, the graphical estimationmethod is capable of intuitively obtaining array detection distances in different operating frequencies, intuitively finding out a variation trend of the detection distances along with the frequencies, and obtaining the optimal operating frequency within different constant threshold intervals.