A heating strap assembly includes a of heating strap engaged with positioning recesses of supports which are connected between two sides of a rectangular frame which includes a first end board, a second end board and the two sides connected between the first and second end boards. Two positioning bars are connected between the first and second end boards to press the supports. An adjusting device includes a U-shaped pull member which is movably connected to the second end board by springs. The pull member is connected to a driving support which is movably connected to the two sides of the frame and includes positioning recesses with which the heating strap is engaged. The springs pull the pull member to pull the heating strap so as to keep the tension of the heating strap.