Abstract: A self-made real-time monitoring system with multiple sensors was applied to acquire audible sound,ultravilot and infrared emission signals in laser welding process.Wedge plates and stepped plates were designed for experiments;RMS,STFT and characteristic frequency-band analysis were used to process signals.It had been found that:the RMS value and the intensity of characteristic frequency band of both audible sound and ultravilot emission are extremely low in the case of excessive penetration;they are much higher when full penetration occurs;for partial penetration,they are distinguishably between the former two statuses.Unstable penetration is noted for its surging signals which are lower than those of full penetration status.Consequently,four distinct penetration statues can be classified obviously.