Abstract: Storage capacity calculation is important to coal mine underground reservoir. According to low efficiency and accuracy of the existing storage capacity calculation method, a precision computing method of coal mine underground reservoir capacity based on CAD is put forward. Firstly, floor elevation data of coal mine underground reservoir can be exacted automatically and precisely, away from the CAD platform, using TeighaX widget of ODA. Floor TIN model of coal mine underground reservoir is constructed based on GIS and TIN. Volume elements of water storage are built with the combination of reservoir level data. By assigning various water storage coefficients to volume elements, preciser and more efficient calculation results of the reservoir capacity are obtained.Based on the above, the WebApi of coal mine underground reservoir capacity calculation with C# .NET platform, the computing WEB platform development of coal mine underground reservoir capacity is developed, which can achieve drawings upload, fast and accurate calculation of coal mine underground reservoir capacity by Web browser. Taking the Underground Reservoir as an example, using the traditional method and the system, the results show that the calculated results of reservoir capacity are basically consistent,The comparison of results shows that the error in A1 area is 0.68% while the error in A3 is 2.66%. Both of them are below 3%, which proves that.which proves that the calculation of the system is reliable.