We　report　a　straightforward　one-pot　synthesis　of　E-3-alkenylindazole　scaffolds　under　mild　reaction　conditions,　involving　successive　processes　of　catalyzed　coupling,　cyclization,　1,2-hydrogen　migration,　and　E/Z　isomerization.　Complemented　by　theoretical　calculations,　the　results　rationalized　the　reaction　mechanism　and　elucidated　that　the　low-energy　light-cyclized　intermediate　C-3a　(T1)　is　the　main　driving　factor　for　the　rare　Z　-＞　E　photoisomerization.　This　synthetic　approach　would　be　an　attractive　and　practical　strategy　for　preparing　diverse　E-3-alkenyl　2H-Indazoles,　owing　to　its　good　substituent　tolerance　and　high　E/Z　selectivity.　Notably,　molecular　engineering　allows　for　the　tuning　of　absorption　and　emission　properties　across　the　visible　spectrum　by　introducing　various　electron-donating　groups　into　specific　styrene　subunits,　making　them　valuable　dyes　for　photonic　or　optoelectronic　applications.