  We report a new search of weakly interacting massive particles (WIMPs) using
the combined low background data sets in 2016 and 2017 from the PandaX-II
experiment in China. The latest data set contains a new exposure of 77.1 live
day, with the background reduced to a level of 0.8$\times10^{-3}$ evt/kg/day,
improved by a factor of 2.5 in comparison to the previous run in 2016. No
excess events were found above the expected background. With a total exposure
of 5.4$\times10^4$ kg day, the most stringent upper limit on spin-independent
WIMP-nucleon cross section was set for a WIMP with mass larger than 100
GeV/c$^2$, with the lowest exclusion at 8.6$\times10^{-47}$ cm$^2$ at 40
GeV/c$^2$.
