Source rocks evaluation is of great importance for gas exploration in Changling depression of southern Songliao basin. Aimed to make clear the TOC distribution of source rocks in Shahezi formation, based on the logging data and core tested data, variable coefficient ΔlgR model was used to calculate the TOC quantitatively, source rocks of different TOC grade and their thickness distribution were described. The key coefficients in the model were calculated using variable coefficient ΔlgR model according to the sampled data of the studied area, and TOC calculation results can reflect the TOC underground accurately. TOC predicting error by variable coefficient ΔlgR model was about 19.1%, which was reduced 17.5% compared to traditional ΔlgR model. Source rocks in Changling depression are characterized by high TOC value, middle less source rocks, good source rocks and excellent source rocks account for 26.1%, 18.2% and 55.7% of all the source rocks in volume, respectively. Good and excellent source rocks mainly distributed in Heidimiao sag, the maximum thickness is over 400 meters, Haerjin and Qianbei sags also developed good source rocks the general thickness is 200~300 meters.