The　real-time　measurement　of　transient　dynamics　of　a　semiconductor　disk　laser　(SDL)　was　demonstrated　using　the　dispersive　Fourier　transform　(DFT)　technology.　Operating　in　a　stable　mode-locking　state　with　a　repetition　rate　of　425　MHz　and　a　pulse　width　of　2.18　ps,　the　SDL　exhibits　significant　changes　in　both　spectral　shape　and　pulse　profile　in　the　process　of　the　establishment　and　extinction　of　mode　locking.　The　pulse-resolved　spectral　evolution　was　captured,　which　includes　relaxation　oscillation,　Q-switched　oscillation　and　mode-locking　states.　Notably,　we　observed　a　progressive　shift　in　the　central　wavelength—from　983.8　nm　during　initial　mode-locking　to　982.5　nm　in　the　stabilization　phase,　and　finally　to　980　nm　at　extinction.　The　shift　of　mode　locked　wavelength　can　be　attributed　to　thermal　effect.　To　the　best　of　our　knowledge,　this　is　the　first　time　to　observe　the　mode-locked　dynamics　in　SDLs　using　a　DFT　technology.　©　2024　Optica　Publishing　Group　under　the　terms　of　the　Optica　Open　Access　Publishing　Agreement.