A free piston type combustion engine comprises a cylinder, a connecting rod and an ignition system, wherein the connecting rod is arranged inside the cylinder, and moves to and fro between two opposed combustion chambers of the cylinder; the ignition system comprises an input signal sensor, an ignition actuator and an electronic control unit; the electronic control unit is in electrical connection with the input signal sensor and the ignition actuator so as to receive signal from the input signal sensor and to control the ignition of the ignition actuator; the input signal sensor comprises a Hall sensor and a linear displacement transducer; and the Hall sensor is arranged on the moving line of the connecting rod of the free piston type combustion engine. When the connecting rod passes, the Hall sensor triggers the linear displacement transducer to carry out accurate induction of the displacement of the connecting rod; moreover, when the displacement reaches to a preset value, the electronic control unit controls the ignition actuator to carry out ignition. The free piston type combustion engine can realize real time judgment of the position of the connecting rod so as to accurately control ignition time and prevent the connecting rod from hitting against a cylinder cover and damaging the free piston type combustion engine.