Plasmon　electronic　dephasing　lifetime　is　one　of　the　most　important　characteristics　of　localized　surface　plasmons,　which　is　crucial　both　for　understanding　the　related　photophysics　and　for　their　applications　in　photonic　and　optoelectronic　devices.　This　lifetime　is　generally　shorter　than　100　fs　and　measured　using　the　femtosecond　pump-probe　technique,　which　requires　femtosecond　laser　amplifiers　delivering　pulses　with　a　duration　even　as　short　as　10　fs.　This　implies　a　large-scale　laser　system　with　complicated　pulse　compression　schemes,　introducing　high-cost　and　technological　challenges.　Meanwhile,　the　strong　optical　pulse　from　an　amplifier　induces　more　thermal-related　effects,　disturbing　the　precise　resolution　of　the　pure　electronic　dephasing　lifetime.　In　this　work,　we　use　a　simple　autocorrelator　design　and　integrate　it　with　the　sample　of　plasmonic　nanostructures,　where　a　femtosecond　laser　oscillator　supplies　the　incident　pulses　for　autocorrelation　measurements.　Thus,　the　measured　autocorrelation　trace　carries　the　optical　modulation　on　the　incident　pulses.　The　dephasing　lifetime　can　be　thus　determined　by　a　comparison　between　the　theoretical　fittings　to　the　autocorrelation　traces　with　and　without　the　plasmonic　modulation.　The　measured　timescale　for　the　autocorrelation　modulation　is　an　indirect　determination　of　the　plasmonic　dephasing　lifetime.　This　supplies　a　simple,　rapid,　and　low-cost　method　for　quantitative　characterization　of　the　ultrafast　optical　response　of　localized　surface　plasmons.