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In this paper,free space communication technology is used in on-line detection system of trains,which provides a good solution for transmission problems between on-site sensors and systemic analysis control.Low density parity check (LDPC) codes are applied for error control.Based on the concepts of complete system of residues system in algebraic member theory,H matrix is constructed.Atmospheric channel model is established while beam expander loss,mismatch loss.atmospheric attenuation,turbulence effects and thermal noise are comprehensively considered.Entire system is established and the simulation result shows that the wireless laser transmission is accurate and reliable and LDPC coding makes the performance improved by 2-3 dB.