Stage treatment system and method for boil-off gas of transport ship

The invention discloses a cargo ship boil-off gas stage treatment system and method, the system comprises a heater, a pressure transmitter, a component detector connected with the heater, a temperature transmitter and a switch valve, and the switch valve comprises a first pipeline valve, a second pipeline valve and a third pipeline valve; the component detector is used for detecting components of output exhaust gas, c0 = 1 is defined to be the component percentage of natural gas in the exhaust gas, and if c0 = 0%, the boil-off gas treatment module is started; if c0 is larger than 0 and smaller than c1%, a nitrogen replacement natural gas processing module is started, and if c0 is larger than c2%, a liquefied natural gas replacement nitrogen processing module is started; the temperature transmitter is used for detecting the exhaust gas temperature T and feeding back the exhaust gas temperature T to the switch valve and the heater, if T is smaller than the preset temperature T0, the heater is turned on, and oth