As one of the main methods to meet the needs of rapid and efficient of urban rail lines, the express/slow train organization mode was very important. But the calculation principle and formula about system capacity loss had some errors. In order to improve the calculation accuracy and application range, the corresponding relationship between departure uniformity and capacity loss was described by introducing uniformity parameter. The capacity loss formula under the mode of slow and fast had summarized and optimized with the premise of uniformity. The accuracy and reliability of the formula were verified by running chart. The main conclusions are as follows：(1)The existing capacity loss formula is based on the principle of capacity maximization, and the uniformity of express trains is not considered in the calculation process, which is quite different from the actual layout results;(2)When the number of fast trains (or slow trains) is constant, the system capacity decreases first and then increases with the evenness; when the evenness is constant, the system capacity decreases with the fast trains (or slow trains) increase;(3)The system capacity loss formula is put forward when the slow train is dominant and the fast train is dominant with the premise of evenness;(4)The optimized capacity loss formula significantly improves the calculation accuracy and application range, which can provide reference for related projects.