In this paper we describe the principle and characteristics of the 2nd harmonic mode-locking of an Ar ion laser. In our experiment, there are two light pulses in an Ar ion laser cavity at any time and the mode spacing for the 2nd harmonic mode-locking of the Ar ion laser is about 210MHz, twice that of the fundamental. This increases the average output power of the Ar ion laser by a factor of two. A theoretical explanation for the observed phenomena is presented.