Abstract: Objective The penetration state of weld is vital for laser-MIG hybrid welding quality. In conjunction with laser-MIG hybrid welding more widely applying in complex structure, the penetration state of weld is unable to determine due to structure’s complexity. Therefore, development of a weld penetration state detection technology is crucial to realize the real-time monitoring of laser-MIG hybrid welding quality. Methods In this paper, a coaxial coupling laser welding penetration detection system based on low-coherence interferometer is developed, aiming at the complex welding conditions of laser-MIG hybrid welding. The depth information of keyhole inside the weld pool is acquired by probe laser. Combined with the filtering algorithm, the real-time weld penetration curve is obtained and the weld penetration state is judged. Results The results show that detection system identifies the weld penetration state accurately. The penetration depth detection error is not more than 2%. Conclusion Through the coaxial coupling laser welding penetration detection system, the real-time monitoring of laser-MIG hybrid welding quality is realized.