A mechanism for radially inserting a yarn into a 3D braided preform, including a first bracket, an upper linear actuator, a support mechanism, an upper yarn insertion device, a yarn carrying device, an upper gear ring, a lower yarn insertion device, a lower linear actuator, a second bracket, a lower gear ring, a lower gear, a lower motor, a lower circular track, an upper circular track, a base, an upper gear, a pneumatic transmission and control device and an upper motor.