Abstract: A shock absorbing seat post structure for a bicycle includes a seat post having a lug defining a pivot hole and an opening located under the pivot hole, a fixed rod mounted in the opening, a supporting base pivotally mounted to the lug and having a front end defining a first receiving space and a rear end pivotally mounted to the pivot hole and defining a second receiving space, a shoulder formed in the supporting base and defining a through hole, an adjusting rod having a front end passing through the through hole of the shoulder and a rear end mounted to the fixed rod, an annular shoulder mounted on the adjusting rod, and an elastic tube having a front end urged on the shoulder of the supporting base and a rear end urged on the annular shoulder of the adjusting rod.