The behaviors of absorption and desorption of light hydrocarbons in marine sediments were studied in laboratory by injecting light hydrocarbon mixed standard gas into the degassing marine sediments. The results of absorption show that the changes of light hydrocarbons are caused by solution and absorption primarily. The results of desorption tests show that thermal desorption can play the role of desorption, and dissolved light hydrocarbons are more prone to desorption than absorbed ones.