In order to accurately carry out the ecological service evaluation of the other forests of the same forest types in the same ecological unit that lacked ecological monitoring data, we propose the forest ecological function correction coefficient based on the biomass in this paper in order to adjust the ecological monitoring data for better application in the rest of the sites of the same forest types in the same ecological unit through the conversion of biomass. As an example, the ecological service of the project of conversion of cropland to forest in Liaoning Province was evaluated. The evaluation showed that the total ecological service value of the project of conversion of cropland to forest in Liaoning Province was 49.194 billion yuan in 2013. In six ecosystem service evaluation indicators such as water conservation, soil conservation, carbon fixation and oxygen releasing, nutrient accumulation, atmosphere purification, and biodiversity protection, the proportions of the value of water conservation and biodiversity protection to the total value were the largest, 26.84% and 26.30%, respectively. Our study would provide reference for the revision of Specifications for assessment of forest ecosystem services and the accurate evaluation of the forest ecosystem service at regional scale.