A tractor assembly includes a connection unit which is connected between the tractor and the wheel chair. The connection unit includes a fixing member, a top link unit, a bottom link unit and a swing rod so as to form a four-link mechanism. The connection unit includes an engaging unit with an engaging section, and a position unit. The top link unit has a reception hole and the positioning unit has a push rod. When the push rod is located at the first position, the reception hole is closed. When the push rod is located at the second position, the push rod is retracted and the reception hole is exposed. The bottom link unit is pivoted about one end of the swing rod to move the other end of the swing rod to the position of the reception hole, the engaging section is inserted into the reception hole.
