The　railway　track　irregularity　is　a　main　influential　factor　to　dynamic　interaction　of　train-track,　and　the　stochastic　process　sample　is　a　indispensable　parameter　in　the　dynamic　model　for　analysis　of　the　train-track.　According　to　stochastic　theory,　the　transformed　relation　of　power　spectrum　density　of　railway　track　irregularity　between　time　and　space　domain　and　the　expression　of　power　spectrum　density　in　time　domain　are　given.　The　principles　and　steps　of　wide-used　method　for　simulation　of　the　train-truck　are　analyzed.　Through　the　analysis　of　numerical　method　about　power　spectrum　density　of　the　railway　track　irregularity,　a　new　method　of　trigonometric　series　is　given,　the　IFFT　and　the　new　method　is　also　equivalent.　Finally　the　numerical　simulation　shows　that　both　the　new　method　and　IFFT　are　more　effective.