Forests play a key role in water conservation. Based on official statistics and field surveys, this paper uses the indirect market evaluation method (IMEM), and especially the water balance method (WBM), to assess the quantity and value of forest water conservation in 2016 in Zhalantun City in Inner Mongolia, China. The results indicate that the value of forest water conservation in Zhalantun is RMB 15.085 billion yuan (RMB is an abbreviated form of ‘Renminbi’, which stands for currency of People's Republic of China. It's primary unit is the yuan), which accounts for 80% of gross domestic product (GDP), and which is 3.74 times the added value of the primary industry, and 1.56 and 2.96 times the added value of the secondary and tertiary industries, respectively, in the same year. The paper also discusses some related issues and points out that the negative effects should also be considered and calculated when using the WBM to evaluate forest water conservation services, and forest water conservation services management and environmental statistics should be established in advance to better serve management planning and policy making.