A saddle adjusting device is slidably disposed on a seat tube and comprises a seat body. The seat body is provided on a top end thereof with a saddle and includes a through portion for insertion of the seat tube. The seat body is provided with at least one seat tube clamp for clamping the seat body on the seat tube and further provided in a middle portion of the through portion with a hollow portion laterally penetrating the seat body and plural engaging grooves. The seat tube is fixedly provided with a bolt for cooperating with the engaging assembly. The engaging assembly can move along the bolt in a direction vertical to the axial direction of the seat tube to make protruding portions of the engaging assembly engage into or disengage from the engaging grooves for adjusting the position of the seat body relative to the seat tube.