  Isomers, ubiquitous populations of relatively long-lived nuclear excited
states, play a crucial role in nuclear physics. However, isomers with half-life
times of several seconds or less barely had experimental cross section data due
to the lack of a suitable measuring method. We report a method of online
{\gamma} spectroscopy for ultra-short-lived isomers from photonuclear reactions
using laser-driven ultra-intense {\gamma}-rays. The fastest time resolution can
reach sub-ps level with {\gamma}-ray intensities >10^{19}/s ({\geqslant} 8
MeV). The ^{115}In({\gamma}, n)^{114m2}In reaction (T_{1/2} = 43.1 ms) was
first measured in the high-energy region which shed light on the nuclear
structure studies of In element. Simulations showed it would be an efficient
way to study ^{229m}Th (T_{1/2} = 7 {\mu}s), which is believed to be the next
generation of nuclear clock. This work offered a unique way of gaining insight
into ultra-short lifetimes and promised an effective way to fill the gap in
relevant experimental data.
