We present a downhole seismic monitoring system and the field test results using a fiber laser seismometer (FLS). The distributed feedback fiber laser is used as the sensing element of the seismometer. Using the interferometric demodulation system, a minimum detectable acceleration of ~ng is achieved. The FLS is installed in a 400-m-depth well in Puer city, Yunnan Province. A micro-earthquake (M=1.2) in Puer area is detected. Compared with the existing underground electrical seismometer, FLS shows a higher signal-to-noise ratio and good reliability, which indicates that the FLS provides a new approach to deep-well seismic monitoring.