A child booster seat includes a base, a flexible connecting member, two attachment parts, and a mechanism. The base has a seat plate for a child sitting thereon. The flexible connecting member is attached to the base. The attachment parts are attached to two ends of the flexible connecting member. The mechanism is disposed on the base for tensing the flexible connecting member. When the child booster seat is secured on a chair through the two attachment parts, the mechanism is operated to tense the flexible connecting member, so that the child booster seat is secured firmly and it is hard to move the child booster seat relative to the adult chair.