The field of plasmonics brings new opportunities in various fields such as photocatalysis, nanophotonics, photovoltaics and photothermal therapeutics. In particular, the generation of hot carriers (hot holes and electrons) as well as the phtothermal effect via excitation of localized surface plasmon resonance (LSPR) can be used to activate chemical reactions. Understanding the underlying mechanism requires quantitative measurements of catalytic enhancement on the plasmonic nanostructures. In this work, the plasmonic gold nanostar (AuNSs) activated electrocatalysis was investigated using ascorbic acid (AA) as a reaction model. We found that AuNSs shows significantly enhanced electrochemical performance upon LSPR excitation. By measurement of the reaction activation energy, open circuit potential (OCP), and the wavelength and light intensity-dependent current responses, it is proposed that the efficient hot carriers separation, reduced activation energy and considerate photothermal effect contribute to the promoted electrocatalysis. Based on the study, a direct plasmon-improved electrochemical sensor towards sensitive detection of AA is constructed, and AA concentration as low as 10.0 mu M can be successfully detected. The present work would shed new mechanistic insights into the plasmon-mediated electrochemical processes. (C) 2019 Published by Elsevier Ltd.