  With the QCD sum rules approach, we study the newly discovered doubly heavy
baryon $\Xi_{cc}^{++}$. We analytically calculate the next-to-leading order
(NLO) contribution to the perturbative part of $J^{P} = \frac{1}{2}^{+}$ baryon
current with two identical heavy quarks, and then reanalyze the mass of
$\Xi_{cc}^{++}$ at the NLO level. We find that the NLO correction significantly
improves both scheme dependence and scale dependence, whereas it is hard to
control these theoretical uncertainties at leading order. With the NLO
contribution, the baryon mass is estimated to be $m_{\Xi_{cc}^{++}} =
3.66_{-0.10}^{+0.08} \text{~GeV}$, which is consistent with the LHCb
measurement.
