  Hawking's area theorem is one of the fundamental laws of black holes (BHs),
which has been tested at a confidence level of $\sim 95\%$ with gravitational
wave (GW) observations by analysing the inspiral and ringdown portions of GW
signals, independently. In this work, we propose to carry out the test in a new
way with the hierarchical triple merger (i.e., two successive BH mergers
occurred sequentially within the observation window of GW detectors), for which
the properties of the progenitor BHs and the remnant BH of the first
coalescence can be inferred from the inspiral portions of the two mergers. As
revealed in our simulations, the BH area law can be well confirmed for some
plausible hierarchical triple merger events detectable in
LIGO/Virgo/KAGRA/LIGO-India's O4/O5 runs. Our proposed method provides
significant facilitation for testing the area law and complements previous
methods.
