The invention provides a method for forecasting an underground crack, which comprises the steps of: obtaining a radial component R(t) and a transverse component T(t) of a converted shear wave; solving a moveout function delta t(t) between a transverse component S1(t) and a slow transverse component S2(t) according to the radial component R(t) and the transverse component T(t); deriving the moveout function delta t(t) between the transverse component S1(t) and the slow transverse component S2(t) to obtain a derivative function dt(t) of the moveout function; and carrying out crack development strength forecasting by using the derivative function dt(t).