Abstract: The present invention is to offer a ratchet diving mask strap which can be adjusted to a desirable tightness even under water through a process of two-piece opposite direction displacement tightening by turning the adjusting knob to tighten the two sides of strap, and consists of one each elastic strap on the left and right, a guard cover an upper rack, a lower rack, a gear, an adjusting knob and a check device, wherein each elastic strap is combined with the left and right sides of the upper and lower racks to form a strip, the upper and lower racks are mutually overlapped and disposed in the guard cover, a gear disposed in the tooth grooves of upper and lower racks to engage with the upper and lower racks, and the gear is combined with the adjusting knob, turning the adjusting knob can displace the upper and lower racks left and right and further pull the left and right elastic strap so as to adjust the tightness of said strap, and the check device can check the adjusting knob to keep the tightness of mask