The invention discloses a method for estimating the user amount in the underlay frequency spectrum sharing manner. The method comprises the following steps that dynamic delayed expansion is implemented on a received signal; singular value deposition is carried out on the expanded signal to obtain a singular value matrix of the expanded signal, and a main diagonal vector of the singular value matrix is extracted; in the main diagonal vector, a latter element is subtracted from a former neighboring element to form a characteristic quantity ch1; in the ch1, a former element is divided by a neighboring latter element to form a characteristic quantity ch2; and the maximal value of the ch2 is searched to estimate the user amount in the underlay frequency spectrum sharing manner. The user amount in the underlay frequency spectrum sharing manner can be effectively estimated under low signal to noise ratio and high frequency spectrum overlapping ratio.