At present, the research on error characteristics and detection methods of individual equipment in smart substation metering system is becoming more and more mature, and the system-level error test method for error association and influence between devices has been validated. However, the modeling and analysis of its comprehensive errors have not been carried out effectively, which makes this type of metering systems lack of analytical basis for error tolerance fitting and disabled the legitimacy of the power-trading purpose of digital metering system. Starting from the factors affecting the metering system error of smart substation, such as sensing, quantization, framing, interpolation, delay, etc., a mathematical model of system composite error is established. And the accuracy of the model is tested and verified in different scenes. The results show that the model is especially suitable for the composite error analysis of metering system in smart substation, which can provide reference for design, test, operation and maintenance of system.