Abstract: The present disclosure relates to a mechanical stretching device for a movable seat unit, and a movable seat unit thereof. The mechanical stretching device includes: a linkage support that comprises a support structure configured to support the mechanical stretching device on ground and a linkage structure attachable to a seating portion of the seat unit; a back mechanism that is pivotally connected to the linkage support and is configured to attach to a backrest of the seat unit; and a leg stretching structure including a footrest element, and the leg stretching structure is pivotally connected to the linkage support, and the footrest element is attached to a footrest of the seat unit. The mechanical stretching device is configured to implement a sequential conversion or a reverse sequential conversion of the seat unit from a sitting state to a relaxing state and to a lying state.