The 2018 tropical cyclone (TC) season over the western North Pacific (WNP) underwent two extreme situations: 18 TCs observed during June-August (JJA) and ranked the second most active summer in the satellite era; only 5 TCs that occurred during September-October (SO), making it the most inactive period since the late 1970s. Here we attribute the two extreme situations based on observational analyses and numerical experiments. The extremely active TC activity and northward shift of TC genesis during JJA of 2018 can be attributed to the WNP anomalous low-level cyclone, which is due primarily to El Niño Modoki and secondarily to the positive phase of the Pacific Meridional Mode (PMM). Overall, the extremely inactive TC activity during SO of 2018 is due to the absence of TC formation over the South China Sea and Philippine Sea, which can be attributed to the in-situ anomalous low-level anticyclone associated with the positive phase of the Indian Ocean Dipole, although the positive PMM phase and El Niño Modoki still hold.