  In the standard model (SM), pair production rate of Higgs boson at the Large
Hadron Collider (LHC) is quite low. One usually think that it is extremely
important for the measurement of triple Higgs coupling at the high luminosity
LHC. In this paper, we propose to search for the extra Higgs boson (denoted as
$S$) utilizing pair production of the SM-like Higgs boson ($H$) which was
discovered in July, 2012.The pair production of $H$ can be huge due to the
resonant production of heavy scalar $S$, namely $PP \rightarrow S \rightarrow
HH$. The couplings of $H$ with weak gauge boson are similar to ones in the SM
and it implies that the couplings between $S$ and gauge bosons are likely
suppressed. Provided that $S$ is heavy enough, the decay into weak gauge bosons
may not be the dominant modes. Instead $S$ can decay into a pair of $H$ and
offer the promising channel to discover it. In this paper, we studied the 5
promising decay modes of $H$, i.e. $b\bar{b}$, $WW^*$, $ZZ^*$, $\gamma\gamma$
and $\tau^+\tau^-$, and simulated the signals and backgrounds for the 15
combination modes for $HH$ at the LHC with $\sqrt{s}= 14$ TeV and integrated
luminosity $\mathcal{L}=1000fb^{-1}$. We found that with the help of suitable
selection rules, very good signal to background ratio $S/B$ can be archived in
many decay channels, for example $b\bar b+ (WW^*, ZZ^*, \gamma\gamma,
\tau^+\tau^-)$, $WW^*+(WW^*, ZZ^*, \gamma\gamma, \tau^+\tau^-)$ and
$\tau^+\tau^- +(\gamma\gamma, \tau^+\tau^-)$. For the detailed results please
refer to Table \ref{tab:discovery} in the text. On the contrary, $b\bar b b\bar
b$ mode is less important due to the huge QCD background. However if one has
excellent control on light jet mis-tagging, the $b\bar b b\bar b$ mode can be
promising to discover the extra Higgs boson due to its the largest branching
ratio compared to other modes.
