The measurement of the electrode current of argon-filled Frank-Hertz (F-H) tube shows that most of the electrons emitted by cathode are absorbed by grid1 and grid2, and only a small fraction forms the plate current. The grid1 and grid2 voltages have a significant influence on the number of electrons emitted by the cathode. Grid1 can not only absorb electrons, but also emit electrons under certain conditions. The direct reason why the F-H curve first increases and then decreases with grid1 voltage is given. The explanation of electron energy distribution with non-zero valley current of F-H curve is also given.