A method for recovering ethylene from off-gas of a process for fluidized catalytic cracking of a heavy oil fraction, the method comprising the processes of: concentrating C2+ components of the FCC off-gas using a pressure swing adsorption process to obtain a C2+ rich mixed gas; and introducing the C2+ rich mixed gas into a displacement desorption process to recover high-concentration ethylene from the mixed gas; wherein the process of concentrating the C2+ components of the FCC off-gas by the pressure swing adsorption process comprises the steps of: i) introducing the FCC off-gas into an adsorption column packed with an adsorbent selectively adsorbing the C2+ components, so that the C2+ components are adsorbed onto the adsorbent and the remaining impurities are discharged out of the adsorption column; ii) compressing a portion of a C2+ rich stream obtained from step iv) and step v) and cocurrently introducing the compressed stream into the adsorption column that was subjected to step i), thereby rinsing impurities away from the adsorption column with a C2+ rich stream pressurized by a compressor so that the inside of the adsorption column becomes a C2+ rich atmosphere; iii) discharging impurities from the adsorption column that was subjected to step ii), by cocurrent depressurization; iv) countercurrently depressurizing the adsorption column that was subjected to step iii), while obtaining the C2+ rich mixed gas; v) recycling a portion of a gas discharged from step i) into the adsorption column that was subjected to step iv), thereby desorbing C2+ components from the adsorption column by low-pressure rinse; and vi) recycling a gas discharged from step i) into the adsorption column that was subjected to step v), and pressurizing the adsorption column to the adsorption pressure; wherein the process of introducing the C2+ rich mixed gas into the displacement desorption process to recover high-concentration ethylene comprises: vii) recovering gases resulting from step xiii) and step ix) into an adsorption column packed with an ethylene selective adsorbent; viii) introducing the C2+ rich mixed gas into the ethylene selective adsorbent-packed adsorption column to adsorb ethylene from the mixed gas, and introducing unadsorbed components and a desorbent, fed into the adsorption column during step x), through the outlet of the adsorption column into a distillation column that separates an ethylene-poor stream/desorbent mixture into components; ix) introducing a high-concentration ethylene, obtained in a distillation column for separating an ethylene rich stream/desorbent mixture into components, into the adsorption column that was subjected to step viii), thereby removing ethane and other gases; and x) introducing a desorbent into the adsorption column that was subjected to step ix) so as to desorb ethylene from the adsorption column, and then sending the desorbed ethylene into the distillation column for separating the ethylene rich stream/desorbent mixture into components, thereby producing concentrated ethylene. i) introducing the FCC off-gas into an adsorption column packed with an adsorbent selectively adsorbing the C2+ components, so that the C2+ components are adsorbed onto the adsorbent and the remaining impurities are discharged out of the adsorption column; ii) compressing a portion of a C2+ rich stream obtained from step iv) and step v) and cocurrently introducing the compressed stream into the adsorption column that was subjected to step i), thereby rinsing impurities away from the adsorption column with a C2+ rich stream pressurized by a compressor so that the inside of the adsorption column becomes a C2+ rich atmosphere; iii) discharging impurities from the adsorption column that was subjected to step ii), by cocurrent depressurization; iv) countercurrently depressurizing the adsorption column that was subjected to step iii), while obtaining the C2+ rich mixed gas; v) recycling a portion of a gas discharged from step i) into the adsorption column that was subjected to step iv), thereby desorbing C2+ components from the adsorption column by low-pressure rinse; and vi) recycling a gas discharged from step i) into the adsorption column that was subjected to step v), and pressurizing the adsorption column to the adsorption pressure; vii) recovering gases resulting from step xiii) and step ix) into an adsorption column packed with an ethylene selective adsorbent; viii) introducing the C2+ rich mixed gas into the ethylene selective adsorbent-packed adsorption column to adsorb ethylene from the mixed gas, and introducing unadsorbed components and a desorbent, fed into the adsorption column during step x), through the outlet of the adsorption column into a distillation column that separates an ethylene-poor stream/desorbent mixture into components; ix) introducing a high-concentration ethylene, obtained in a distillation column for separating an ethylene rich stream/desorbent mixture into components, into the adsorption column that was subjected to step viii), thereby removing ethane and other gases; and x) introducing a desorbent into the adsorption column that was subjected to step ix) so as to desorb ethylene from the adsorption column, and then sending the desorbed ethylene into the distillation column for separating the ethylene rich stream/desorbent mixture into components, thereby producing concentrated ethylene.