Abstract: A system for the monitoring a preload in a ball screw includes a ball screw, a detecting device, a capturing device and a processing device. The ball screw has a preload. The detecting device detects an operating signal of the ball screw. The operating signal contains information about the ball screw's rotating speed and status. The status refers to a vibration or a sound generated by the operating ball screw. The capturing device is connected to the detecting device and acquires the operating signal. The processing device is connected to the capturing device, and has a threshold. The processing device processes and converts the operating signal into a ball-passing signal. When the ball-passing signal surpasses the threshold, the processing device determines that the ball screw's preload has vanished. Thereby, the system is enabled to determine the current state of the ball screw's preload reliably.