The invention relates to a treatment method for dehydrating and upgrading high-moisture and high-volatile low-rank coal, and discloses a method for preparing blast furnace blowing semicoke by upgrading low-rank coal based on a high-pressure hydrothermal reaction, wherein the method comprises four steps of low-rank coal pretreatment, high-pressure hydrothermal upgrading, waste heat recycling and upgraded semicoke dehydration and drying. According to the method, low-heat-value low-rank coal is converted into high-quality blast furnace blowing upgrading semicoke by utilizing a high-pressure hydrothermal reaction upgrading technology, wherein the moisture content of the semicoke air drying base is less than 6%, the volatile component content is less than 25%, the Hardgrove grind ability indexis greater than 60, the high-order heat generation value is greater than 24000 kJ/kg, soluble potassium salt, sodium salt and sulfate components in ash are dissolved out, the organic sulfur in coal iseffectively separated out, and the upgraded semicoke can meet the performance index requirements of coal for blast furnace blowing; and the method provided by the invention effectively solves the problems of high water content, high volatile component content, low heat value and high harmful element content of blast furnace blowing of low-rank coal, and provides a clean, efficient, low-energy-consumption and high-added-value low-rank coal upgrading utilization method.