A safe discharge method for waste lithium ion batteries includes steps of mixing the waste lithium ion batteries and conductive particles in a discharge chamber to make the waste lithium ion batteries to discharge, calculating an internal resistance of the discharge chamber according to pressurization pressure; calculating a discharge rate of the waste lithium ion batteries; dynamically adjusting the pressurization pressure to keep the discharge rate of the waste lithium ion batteries to be 0.1-3 C; monitoring an internal temperature of the discharge chamber in real time; when the internal temperature is greater than an early warning temperature, reducing the pressurization pressure by 20%-60%; when the internal temperature is greater than a warning temperature, relieving the pressurization pressure to 0 N, reducing the pressurization pressure by 60%-90% after the internal temperature drops below the early warning temperature, and re-compacting to discharge the waste lithium ion batteries.