The invention belongs to the technical field of metal joining and particularly relates to an inertial friction welding method based on preheating by friction. The inertial friction welding method comprises the following steps: 1) firstly, fixing workpieces to be welded respectively through a rotary fixture and a mobile fixture, contacting the workpiece at a mobile end with the workpiece at a rotary end, applying a pre-tightening force to the workpieces, driving the rotary end to rotate, and preheating welding surfaces of the workpieces, wherein the pre-tightening force cannot be greater than afriction pressure in the inertial friction welding process; 2) after the rotation speed of the rotary end reaches a set rotation speed, increasing the applied pre-tightening force to the friction pressure, and disconnecting the connection between a spindle and a flywheel to allow the rotary end to continue rotating by the inertia of the rotary end; and 3) after the amounts of shortening, caused by friction, of the workpieces, reach a set value, or after the rotation speed drops to a set value due to friction, increasing the applied friction pressure to a upsetting force, maintaining the pressure, and ending the welding. The inertial friction welding method of the invention widens the parameter window of the inertial friction welding process, effectively utilizes the welding capability offriction welding, and further improves the industrial application value of the friction welding technology.