The invention discloses a selection and automatic protection device for a drill stem of a horizontal directional drilling machine. The selection and automatic protection device comprises a controller (1), a rotary pressure sensor (2), a rotary handle (3) and a displayer (4); the rotary pressure sensor (2) is used for monitoring the pressure in a power head rotary motor in real time; the rotary handle (3) is used for achieving left-and-right rotation of the drill stem, and the drill stem is connected with a drill collar at the front portion of a power head; the controller (1) is connected with the rotary pressure sensor (2), the rotary handle (3), the displayer (4), a positively-rotating electromagnetic valve (5) of a rotating pump (7) and a negative-rotating electromagnetic valve (6) of the rotating pump (7). By means of the selection and automatic protection device, according to the model and the maximum torque of the drill stem selected by a user, the overflow pressure converted through the maximum torque of the utilized drill stem is selected on the displayer, safety construction of the drill stem of the drilling machine is accordingly guaranteed through real-time detection of the rotary pressure sensor, and therefore energy conservation and environmental protection are achieved; through pressure measurement and pressure limitation of the power head rotary motor, selection and automatic protection of the drill stem of the horizontal directional drilling machine are achieved.