A knee protective assembly has a mounting device and a protective device. The mounting device has two mounting frames and two jointing elements. Each mounting frame has a mounting panel, two articulated arms, a connecting tab and an adjusting mount with multiple adjusting holes. The jointing elements are respectively connected to the articulated arms of the mounting frames. The protective device is detachably connected to the mounting device and has two protective pads. The protective pads are respectively connected to the mounting frames of the mounting device and each protective pad has two inserting rods and two fasteners. Accordingly, after loosening the fasteners from the inserting rods on the upper protective pad and inserting the inserting rods into another adjacent adjusting holes of the upper mounting frame, the length of mounting device of the knee protective assembly is adjusted to fit with different users' legs.