The western tropical Pacific (WTP) exhibits large interannual sea level anomalies (SLAs), and the sea level falling in El Niño is evidently stronger than the rising in La Niña. The asymmetry is most prominent near 160°E with the response to El Niño larger by three times and becomes less obvious near the western boundary. Sensitivity experiments of a simplified ocean model suggest that the asymmetry in surface wind forcing structure between El Niño and La Niña is critical. The El Niño’s westerly wind anomaly patch locates more east than the La Niña’s easterly wind patch during the mature stage, and its upwelling effects are accumulated over a wider longitude range and cause stronger negative SLAs in the WTP. Near the western boundary, however, upwelling effects are attenuated by easterly wind anomalies during El Niño conditions. The asymmetric ocean responses to ENSO winds may participate in the asymmetry of ENSO cycle.