This research investigates the flexural behavior of steel-concrete composite floor slabs with distributed fiber optic sensors based on pulse pre-pump Brillouin optical time domain analysis. The distributed sensors were embedded in trapezoidal concrete slabs and provided in-situ real-time monitoring of initiation and evolution of cracks in the profiled slabs of four composite floor slabs. This paper proposes a sensor deployment method for real construction of civil engineering structures and evaluates the effects of headed studs and concrete casting on the flexural behavior of composite floor slabs and the performance of distributed sensors. The composite floor slabs failed with concrete cracks, accompanied by yielding in the steel beams and slippage between concrete and steel. The distributed sensors captured complex crack patterns of the concrete slabs in the evolution of the damages in the composite floor slabs. The strengths and limitations of distributed sensors are discussed to promote applications and further development.