The invention discloses an L-shaped external gear pump suitable for an ultralow-viscosity medium. The provided L-shaped structure for realizing lateral flow-in of the medium and axial flow-out at a rear end aims at the design requirements of ultralow viscosity of 0.00018 Pa.s and high volumetric ratio of above 90%, so damages of convectional later outlets to the strength and the rigidity of a pump shell are avoided, and the thickness of the pump shell is properly reduced. Optimal gear pair parameters make the pump have a smallest size under given design conditions and reduce the spacecraft launching cost. The shape and position dimensions of a rear end medium outlet accord with circular unloading groove design, and also has an outlet side pocketed oil unloading function, A large included angle of an inlet compensates the cavitation phenomenon caused by insufficient self suction, and combination of a small included angle of the outlet with automatic compensation of a floating shaft sleeve effectively controls axial leakage and indirectly improves the volumetric ratio. Compared with convectional ground oil pumps, the pump disclosed in the invention has the profile characteristics of small axial dimension and large radial dimension.