The system error refers to that under the same condition and during the process of repeatedly measuring the same measuring value, the absolute value and simbol occuring keep unchanged or change at a certain regulation with the condition changing. 

The system error is caused by the reasons of that the measurement tool proper is inaccurate or the measurement tool is not correctly installed or adjusted, the measuring person's visual capacity or inherent reading habits and the theory of measuring methods have flaws or the approximation formula is adopted. For the system error with known absolute value and symbol, it can be eliminated by introducing the modification value, that is to say, add one numerical value with the same size error and with contrary symbol to the system in the measurement results, in order to compensate the impact of the system error on measurement results. During the process of measuring, people can take the appropriate measurement methods to reduce or eliminate the system error as possible.