The invention discloses a cantilever gravity anchor used in carbonate rock and a soil sea area: a cantilever is arranged on the front end surface of the gravity anchor main body, the cantilever widthis equal to the width of the gravity anchor main body, the truncated surface is a right-angle trapezoid, the upper end surface is an inclined surface, the lower surface is a plane flush with the lowersurface of the gravity anchor main body, and the lower surface is provided with shear keys at equal intervals. On this basis, a cantilever length design method of cantilever gravity anchor is proposed, including: constructing a finite element file; after finite element calculation, drawing a horizontal tension- displacement relation curve, and obtaining the peak resistance and allowable resistance of traditional conventional gravity anchor model; changing the cantilever length, and repeating the above to obtain the peak resistance and allowable resistance of the cantilever gravity anchor model; drawing the curves of different cantilever length growth rate and cantilever gravity anchor peak resistance growth rate , and the curves of different cantilever length growth rate and cantilever gravity anchor allowable resistance growth rate are plotted; determining the cantilever length and designing and calculating the cantilever gravity anchor model.