A computer simulation is carried out to evaluate the accuracy of the ion temperature determined from the collective Thomson scattered spectrum, when a high-power, multimode D2O laser beam is incident. The simulation results reveal the per cent bias error B<1% and per cent standard deviation D<6% when a single-mode incident beam of duration tau =1 mu s is scattered by a plasma with ne=1013 cm-3, Te=Ti=300 eV, and the scattered spectrum is detected with a frequency resolution Delta f=10 MHz. The values of B and D increase considerably when the scattered spectrum for a multimode incident beam is fitted with theoretical spectrum for single-mode incidence. However, they reduce to almost the same values as those for the single-mode incident beam when the theoretical spectrum for the same multimode incidence is fitted in estimating Ti.