In this peper, we investigate the possibility of generating the ultrashort optical pulse in far-infrared free electron laser driven by a radio-frequency linear accelerator (RFLinac). The short electron pulse emits the optical pulse the length of which is correspondent with the length of the electron PUlse. We Present an equivalent refractivity to describe the Propegation of the optical pulse. When a moderate original Phase velocity of the electron pulse is chosen to ensure that the gum be POsitive and the equivalent refractivity monotonously increase along the relative slip distance, the leading and trailing edges will relativiely move toward the center of the optical pulse during interaction. Thus, the high-power and ultrashort optical pulse in the far-infrared region would be generated.