An engaging mechanism, comprising: at least one rotary fastening element rotating about an axis to reach an engaging state; at least one guiding frame having two opposing outer sidewalls (310, 320) and being protrudingly disposed on a support surface of a transmission dock, wherein the axis is substantially parallel to the support surface, and the at least one rotary fastening element protrudes from one of the two outer sidewalls (310, 320) in the engaging state; and at least one translating frame having two opposing inner sidewalls (410, 420) and two upper wall surfaces (430, 440) adjoining the two inner sidewalls (410, 420), respectively, wherein, when the at least one translating frame connects with the support surface, the two outer sidewalls (310, 320) are disposed between the two inner sidewalls (410, 420), and the at least one rotary fastening element rotates and reaches the engaging state to thereby stop one of the two upper wall surfaces (430, 440), wherein the at least one rotary fastening element comprises a pivotal portion, an arm, a hook portion, and a rod, the pivotal portion being positioned proximate to a side of the at least one guiding frame through the axis, allowing the arm and the hook portion to extend from the pivotal portion successively, wherein the hook portion protrudes from one of the two outer sidewalls (310, 320) and is stopped at one of the two upper wall surfaces (430, 440), when the at least one translating frame connects with the support surface and the at least one rotary fastening element is in the engaging state, wherein the engaging mechanism further comprises a linking element and an elastic element, wherein the linking element connects with the two rotary fastening elements, and the elastic element connects with the linking element whereby normally the two rotary fastening elements reach the engaging state; and wherein one of the two inner sidewalls (410, 420) pushes a pushed area of the hook portion and causes the at least one rotary fastening element to rotate counterclockwise and quit the engaging state while the at least one translating frame is moving in a support direction substantially perpendicular to the support surface so as to connect with the support surface; wherein when the connecting lateral side connects with the support surface, the translating frames no longer press against the hook portion of the rotary fastening element, and in consequence the elastic element pulls the rod of the rotary fastening element in a direction through the linking element, thereby allowing the rotary fastening element to turn clockwise and reach the engaging state. at least one rotary fastening element rotating about an axis to reach an engaging state; at least one guiding frame having two opposing outer sidewalls (310, 320) and being protrudingly disposed on a support surface of a transmission dock, wherein the axis is substantially parallel to the support surface, and the at least one rotary fastening element protrudes from one of the two outer sidewalls (310, 320) in the engaging state; and at least one translating frame having two opposing inner sidewalls (410, 420) and two upper wall surfaces (430, 440) adjoining the two inner sidewalls (410, 420), respectively, wherein, when the at least one translating frame connects with the support surface, the two outer sidewalls (310, 320) are disposed between the two inner sidewalls (410, 420), and the at least one rotary fastening element rotates and reaches the engaging state to thereby stop one of the two upper wall surfaces (430, 440), wherein the at least one rotary fastening element comprises a pivotal portion, an arm, a hook portion, and a rod, the pivotal portion being positioned proximate to a side of the at least one guiding frame through the axis, allowing the arm and the hook portion to extend from the pivotal portion successively, wherein the hook portion protrudes from one of the two outer sidewalls (310, 320) and is stopped at one of the two upper wall surfaces (430, 440), when the at least one translating frame connects with the support surface and the at least one rotary fastening element is in the engaging state, wherein the engaging mechanism further comprises a linking element and an elastic element, wherein the linking element connects with the two rotary fastening elements, and the elastic element connects with the linking element whereby normally the two rotary fastening elements reach the engaging state; and wherein one of the two inner sidewalls (410, 420) pushes a pushed area of the hook portion and causes the at least one rotary fastening element to rotate counterclockwise and quit the engaging state while the at least one translating frame is moving in a support direction substantially perpendicular to the support surface so as to connect with the support surface; wherein when the connecting lateral side connects with the support surface, the translating frames no longer press against the hook portion of the rotary fastening element, and in consequence the elastic element pulls the rod of the rotary fastening element in a direction through the linking element, thereby allowing the rotary fastening element to turn clockwise and reach the engaging state.