The present invention discloses a pen having a swinging decoration. A top end of the pen is installed with an animal-shaped decoration. A mandible and a tail section of the decoration are movable. When a user writes with the pen with a small applied force or the pen is in a standstill state, the mandible and the tail section are at an upper location. When the user writes with the pen with a large applied force, a refill will push an activity rod upward and compress a spring, allowing a tooth row at an upper and lower end of the activity rod to drive a left and right conjugate gear and a gear on the decoration to rotate at a same time, which further enables the mandible and the tail section to swing downward simultaneously.