The invention discloses a portable low-current grounding line selection tester and a test method thereof. The tester comprises a shell. A surface of the shell is provided with a power supply jack, a power supply switch, a voltage output jack, a current output jack, a voltmeter, an ammeter, a voltage-regulating rotary knob and a current-regulating rotary knob which are connected to a test circuit in the shell. The test circuit comprises a transformer. A main edge of the transformer is connected to a power supply and simultaneously is connected to the power supply switch. A secondary side output of the transformer is two sides. An upper side is connected to the voltmeter and an output is an adjustable alternating voltage. A lower side is connected in series with the ammeter and an output is an alternating current. There are three output branches. One branch is connected in series with a slide rheostat and then is reserved as a current output jack. The other two branches are directly reserved as the current output jacks. The tester is specially used for low-current grounding line selection apparatus testing. The size is small, weight is light, wiring is simple and the tester is portable. Testing efficiency of low-current grounding line selection is increased and labor intensity of a maintainer is mitigated.