In this paper, we studied a kHz nanosecond 1645nm OPO for methane detection. The OPO pump source was an electro-optical Q-switched 1064 nm oscillator, followed by a preamplifier and two Innoslab amplifiers. The nonlinear working material was two KTA crystals with type II angular phase matching, and two plane mirrors were used for the OPO cavity. The signal light with an average power of 9.32 W and the minimum pulse duration of 1.8 ns was achieved at a repetition rate of 8 kHz for 54.1 W pump power, and the optical-optical conversion efficiency was 17.3%. The average beam quality was measured to be M2=1.15. The wavelength of the signal light was continuously tunable from 1641.9207 nm to 1648.1791 nm. To the best of our knowledge, this was the highest average power achieved at kHz regime of 1645 nm laser.