Abstract: The studies of complex lithology reservoir mostly was confined to carbonate rocks at home and abroad, and the researches aimed to granite reservoir evaluation, especially the method for estimation while drilling also lacked of systematization and quantification. This paper, for the first time, introduced work index model to assess granite reservoir. According to the in?depth analysis of the model, the concrete steps for debugging of key coefficients of the model, drawing of work index trend line and calculation of work index ratio were elaborated, thus establishing a qualitative reservoir identification method while drilling based on work index ratio. Meanwhile, by calibrating work index ratio from the results of imaging logging, the evaluation criterion of reservoir property was established, achieving the semi?quantitative assessment while drilling for reservoir property for the purpose of providing certain references for preventing engineering risks such as leakage and borehole collapse. The application results show that this method is more accurate than single drilling time recognition method for granite reservoir identification, and consistent with the comprehensive log interpretation results as required in field exploration.