The invention discloses a steam turbine low-pressure through-flow area safety monitoring and prewarning system and a working method. Data is acquired through a flow area air blowing monitoring module,a long blade vibration monitoring module, a blade water erosion water brush prewarning module and an optimal operation boundary adjusting module; and after intercommunication, target unit safety boundary data and optimal safety operation boundary are generated in a safety estimating module and the optimal operation boundary adjusting module to prompt safe and economical adjustment of an unit. Themethod is suitable for steam turbine low-pressure through-flow area safety monitoring and prewarning in operation of a deep peak adjusting unit under part-load working condition, can accurately, comprehensively and effectively predict the low-pressure through-flow area long blade air blowing critical steam inlet flow and the air blowing high-temperature area position in deep peak adjustment, canfinely monitor the long blade vibration stress peak magnitude, can prospect and prewarn the water erosion state of final-stage blades, and achieves further optimization to give the safety operation boundary to the unit.