In order to fabricate a microcavity with a complex structure on the surface of stainless steels, the electrochemical micromachining technology based on fiber laser surface modification is adopted. First, the 304 stainless steel surface is masked with a predefined pattern by using a fiber laser. Then, by virtue of the corrosion resistance property of laser masked layers, a microcavity can be locally machined by the electrochemical etching technology. A composite machining system based on electrochemical micromachining in combination with fiber laser masking is developed. The system parameters are optimized through process comparison experiments by using 304 stainless steel as specimens. The results show that a microcavity with any typical structure can be fabricated quickly with this system.