  Precise measurement of two-neutrino double beta decay (DBD) half-life is an
important step for the searches of Majorana neutrinos with neutrinoless double
beta decay. We report the measurement of DBD half-life of $^{136}$Xe using the
PandaX-4T dual-phase Time Projection Chamber (TPC) with 3.7-tonne natural xenon
and the first 94.9-day physics data release. The background model in the
fiducial volume is well constrained in situ by events in the outer active
region. With a $^{136}$Xe exposure of 15.5 kg-year, we establish the half-life
as $2.27 \pm 0.03 (stat.)\pm 0.10 (syst.)\times 10^{21}$ years. This is the
first DBD half-life measurement with natural xenon and demonstrates the physics
capability of a large-scale liquid xenon TPC in the field of rare event
searches.
