A knee joint protecting device includes a thigh frame connected to a shank frame by a connection structure, which includes an adjustment mechanism and a connection hinge. The thigh frame is rotatable around a first axis using the adjustment mechanism to perform a first rotation about the shank frame within a first position and a second position. When the thigh frame is in the first position, the knee joint protecting device is fit for the lower limb of a standing posture. The thigh frame performs a second rotation about the shank frame using the connection hinge, the included angle of the first axis and the rotation plane of the second rotation is within the range of 0˜15°. By the adjustment mechanism and the connection hinge, the knee protecting device can provide the internal and external rotations between the thigh and the shank, and is more suitable for human engineering and kinesiology.
