The invention discloses a variable-diameter blowout preventer for a controllable non-well killing operation. The variable-diameter blowout preventer comprises a gate assembly and a slip assembly, wherein the gate assembly comprises a gate body and a variable-diameter front seal; the variable-diameter front seal comprises a rubber body, a front seal skeleton and a floating sizing block; the rubber body is fixedly connected with the gate body; a first semicircle gap is formed in the front end of the rubber body; the front seal skeleton is fixed at the upper surface and the lower surface of the rubber body; a second semicircle gap which is concentric with the first semicircle gap is formed in the front end of the front seal skeleton; the diameter of the second semicircle gap is greater than that of the first semicircle gap; the floating sizing block is clamped between the upper surface of the rubber body and the corresponding front seal skeleton; the floating sizing block and the rubber body are fixedly connected and can move horizontally and slide relative to the front seal skeleton; and a third semicircle gap which is concentric with the first semicircle gap and the same as the first semicircle gap in diameter is formed in the front end of the floating sizing block. The variable-diameter blowout preventer can adapt to the clamping requirements of two tube pillars with different diameter specifications; a gate and a slip do not need to be replaced on the spot; and the variable-diameter blowout preventer is convenient to use.