In order to investigate the effect of the overburden''s thickness in riverbed on the stability of the earth-rockfill dam''s slope, the work of computation and analysis have been done with strength reduction finite element method. The safety factors of slope stabiity are calculated when the overburden''s thickness is 0,5, 0,5 and 20m respectively, and the other conditions are the same. The results show that the thicker the overburden is, the less the safety factor is. However, when the overburden''s thickness reaches 1 time of the dam''s height, the safety factor keeps invariable, and will not become less when the overburden becomes thinker.