A webbing length adjustment device has an outer shell, an adjustment assembly, and an elastic member. The adjustment assembly is disposed in and is moveable relative to the outer shell, and has an operating member having a lateral pressing plate portion and a sliding member moving relative to the operating member. The lateral pressing plate portion is formed on the operating member, is located above the outer shell, and is located beside the second through hole for giving more areas to exert force easily and operate conveniently. The lateral pressing plate portion is located at an outer side of a first webbing. There is no interference between the first webbing and user's hand. The elastic member is disposed in the outer shell, and is connected to the outer shell and the operating member for giving a restoring force to the operating member. The outer shell is hard to damage.