A cushion structure for a bicycle contains a body and a bottom tube. The body includes a circular inserting seat. The inserting seat includes a plurality of crossed support ribs extending radially from a center thereof and connecting with an outer rim thereof and formed in a cross shape. The bottom tube includes a connecting portion disposed on one end thereof. The connecting portion includes a circular inner hole corresponding to the inserting seat and includes a plurality of retaining slots formed therein corresponding to the support ribs.