The invention belongs to the technical field of steelmaking and discloses a method for controlling fluctuation of the liquid level of a continuous casting crystallizer. The method includes the steps that covering slag is put to the liquid level of the crystallizer to form a covering slag layer. A flat plate is laid on the covering slag layer. The face, in contact with the covering slag layer, of the flat plate is a plane, and the flat plate is kept located on the covering slag layer and does not sink into the covering slag layer. The method is directly used for solving the problem that fluctuation of the liquid level in the crystallizer is too large; due to the cooperation of the covering slag layer and the flat plate, manners of buffering and gradual weakening are achieved, fluctuation is inhibited in a forced manner, efficiency is high, and operation is easy and convenient.