The precision measurement of vertical tank volume is one of key problems for the international trade of liquefied petroleum products. One measurement system based on Electro-Optical Distance-Ranging (EODR) principle was proposed. Laser ranging and optic-encoding methods were applied to determine the location of each point on the tank shell. Based on the coordinate computation, the space model of vertical tank was established. Regarded as cylinder, the volume of tank can be calculated by using fitted radius of each course. Weighted Average Method and Direct Iterative Method were used to carry out radius fitting. One comparison experiment was designed, in which one 1000m3 vertical tank was used as test object, and Strapping Method was regarded as reference according to OIML R71. The maximal radius errors of two methods were 2.05mm and 1.48mm, and the absolute value of mean radius errors were 0.97mm and 0.63mm, which verified the system discussed.