Biological chemical defense mask

The utility model discloses a biological chemical defense mask which comprises a mask cover, an air inlet pipe is communicated and fixed to the bottom of the mask cover, a connecting box is fixedly connected to the bottom of the air inlet pipe, a circular hole is formed in the inner top wall of the connecting box, a circular block is fixedly arranged in the circular hole in a sleeved mode, and a circular-truncated-cone-shaped hole is formed in the bottom of the circular block. The circular-truncated-cone-shaped hole communicates with an air inlet pipe, and a screw rod is rotationally connected to the inner wall of the right side of the connecting box. A rotary knob is rotated to drive a moving block to move leftwards to extrude a connecting rod through rotation of a screw rod, the connecting rod drives a transverse plate to move upwards to compress two springs, the transverse plate drives a circular-truncated-cone-shaped block to move upwards, and along with movement of the circular-truncated-cone-shaped bloc