There was a peculiarly strong heat-wave event in May 2018 in Zhejiang Province. The extreme maximum temperature (EMT) in most areas was higher than 36 ℃ and in a few regions it could reach 40 ℃. The EMT and high-temperature days (NHD) in many stations broke historical record. The accumulated station-high-temperature days and effective accumulated high temperature both ranked the first in observational record. Based on daily observations at 66 normal stations in Zhejiang Province, NCEP/NCAR reanalysis data, and circulation indices of western Pacific subtropical high (WPSH) provided by National Climate Center, a diagnostic analysis was conducted to illustrate the large-scale circulation cause for such extreme high-temperature event. The results indicate that an enhanced, northward and westward western-distributed WPSH, which accompanied by eastward-march of the south-Asia high, was the direct reason for such extreme heat-wave event. Under the control of WPSH, broad anticyclonic horizontal wind anomaly formed in the region of southeast China-northwest Pacific on 850 hPa, and downward flow prevailed over Zhejiang region. During 14-18 of May 2018, positive anomaly of short-wave radiation flux and the subsident air worked together to intensify the extremity of prominent heat-wave. The change of intensity and location of WPSH was intimately linked with tropical and middle-latitude circulation. More supervital convection in Marine Continent, tropical middle Pacific and north Indian Ocean, weakened convective activity in south China Sea, decreased typhoon in tropical western Pacific acted jointly to enhance WPSH, while in mid-latitude, northward-placed westerly jet on 200 hPa made for stable maintenance of WPSH.