Abstract: Some uniaxial crystals, such as KDP,DKDP, are usually used as Q-switch in laser technology. In practical application, the determination of the x(y) axis and the half wave voltage of uniaxial crystals can provide a great convenient for adjusted experiment of laser. In this paper, a rapid simple and convenient determination method of the x(y) axis and the half wave voltage of uniaxial crystals has been introduced.