The　influence　of　the　coherent　artifact　in　a　semiconductor　Ga-doped　ZnO　film　on　femtosecond　pump-probe　measurement　is　studied.　The　coherent　artifact　mixed　into　the　pump-probe　signal　can　be　directly　inspected　by　detecting　the　background-free　first-order　diffraction　signal　induced　by　the　interference　between　the　pump　and　probe　pulses.　Experimental　results　show　that　by　varying　the　polarization　angle　or　adjusting　the　relative　intensity　between　the　pump　and　probe　pulses,　the　coherent　artifact　can　be　eliminated　from　the　pump-probe　measurement.