Injection pen and dosage adjusting mechanism thereof

The invention discloses an injection pen and a dose adjusting mechanism thereof. The dose adjusting mechanism comprises a knob, a stop support and a torsional spring support. The knob can drive the stop bracket to rotate relative to the torsional spring bracket; the stop support is provided with a cantilever, the torsion spring support is provided with an inner ratchet ring, and the cantilever and the inner ratchet ring are in one-way rotating fit in the dose adjusting direction. The knob is provided with an arm part positioned on the outer side of the cantilever; the rotary knob can also rotate relative to the stopping support so as to be switched between a first position and a second position, in the first position, the rotary knob can directly drive the stopping support to rotate in the dose adjusting direction, the cantilever is in one-way rotating fit with the inner ratchet ring, and in the second position, the arm part inwards presses and abuts against the cantilever in the radial direction so that the