The invention discloses a bootstrap system capable of guaranteeing lignite boiler dry output and SCR safe flue gas temperature. The bootstrap system comprises a boiler tail flue and a flue gas denitrification SCR which is arranged at the tail end of the boil tail flue. A bypass flue communicated with the boil tail flue is further parallelly connected between a flue gas inlet of an economizer and a flue gas inlet of the flue gas denitrification SCR. The bypass flue is internally and sequentially provided with a primary-air tubular heater and a low-pressure economizer which are used for lowering temperature of flue gas entering the flue gas denitrification SCR. Due to the arrangement that the bypass flue is additionally arranged on the tail portion of a boiler, and the primary-air heater is arranged on the upper portion of the bypass flue, primary-air temperature is ensured to meet requirements of a coal pulverizing system. The amount of flue gas passing through the economizer can be regulated by the bypass flue, so that flue gas temperature at the exit of the economizer can be regulated. The primary air is heated to the temperature as required by the high-temperature flue gas at the inlet of the economizer, and meanwhile, the flue gas temperature in the bypass flue is lowered through the low-pressure economizer, so that the flue gas temperature at the inlet of the SCR can meet the requirements of catalyst.