The invention relates to the technical field of radial horizontal rotating drilling and in particular relates to a radial horizontal rotating drilling flexible shaft protector. The connecting cylinder of the protector is hooked on a step on the outer wall of a motor joint and locked by use of a safety pin; a pin protective jacket is arranged outside the safety pin; corresponding pressure relief holes are formed in the connecting cylinder and the motor joint, respectively; a pressure relief sleeve is connected to the outer side of the motor joint by use of left-hand threads and the pressure relief hole of the motor joint is sealed by the pressure relief sleeve; a pressure relief guide pin is mounted on the sidewall of the connecting cylinder; the bottom end of the pressure relief guide pin extends into the axial slide of the pressure relief sleeve; the lower end of the pressure relief sleeve is an annular space formed by the lower part of the motor joint and the connecting cylinder. The radial horizontal rotating drilling flexible shaft protector realizes the prevention of complex underground conditions such as formation drilling flexible shaft twisting-off or deformation, drill bit falling, jamming of the drill, and impedance and blockage to the pulling out of the tool due to overlarge torque transfer, effectively protects the formation drilling flexible shaft and improves the construction efficiency.