The invention relates to a method for cutting off a thick-walled pipe 3, wherein the pipe to be cut comprises an inner circumference closer to the longitudinal axis of the pipe and an outer circumference further from the longitudinal axis of the pipe. The method comprises bringing the cut-off point of the pipe to the cut-off point of a sawing unit, locking the pipe in its place by means of a fastener, sawing the pipe at the cut-off point by moving a saw 2 of the sawing unit towards the center of the pipe 3, the sawing unit comprising at least two saws. The method is characterized by moving each saw 2 of the sawing unit during the sawing radially towards the centre of the pipe until the saw blades pass through the inner surface of the pipe, and rotating the pipe during the sawing with respect to its longitudinal axis by means of a rotating device 10 connected to the fastener until the pipe is cut off, whereby, due to the rotating of the pipe, the blades 25 of the saw 2 work the pipe during a blade revolution only in one direction, which is directed from the outer circumference towards the inner circumference. Furthermore, the invention provides a saw assembly and a rotating device for implementing the method.