A harmless treatment and regeneration device and method thereof of waste liquid discharge from failed hardener solution during hardening of water glass mold shell via ammonium chloride solution are provided. The device includes a waste liquid storage tank, a filtering mechanism, a regeneration mechanism, and a regeneration liquid storage tank. A fluid outlet end of the waste liquid storage tank is in fluid communication with a fluid inlet end of the filtering mechanism, a fluid outlet end of the filtering mechanism is in fluid communication with a fluid inlet end of the regeneration mechanism, and a fluid outlet end of the regeneration mechanism is in fluid communication with a fluid inlet end of the regeneration liquid storage tank.