Abstract: Laser threshold power density of hot isometric pressure(HIP) beryllium in laser beam welding was investigated.The results show that the threshold power density was sensitive to the diameter of beryllium cylinder.It is seemed that more large diameter of beryllium cylinder, more power of back-reflection light coupling into the resonator.So more strong Q-switched pulse was formed in the cavity.Then threshold power density of HIP beryllium will decrease. For the same diameter of bery llium cylinder, when the angel of incident laser beam was adjusted from 0° to 3°, the back-reflection laser can be overcome.Threshold power density of HIP beryllium will increase.In experiment, the ratio of depth to width usually is 1.0-1.5 for deep penetration welding.However, the ratio of depth to width usually is 0.2 for heat conduction welding.