A system and method for calculating an observational error of a coordinate measuring machine includes receiving measuring parameters input by a user, obtaining observational errors of the ruler from a storage system, and identifying the zero mark of the ruler. A direction of the ruler is adjusted to be substantially parallel to the measuring direction of the measuring parameters, controlling the coordinate measuring machine to measure the ruler according to the measuring parameters, obtaining measured data. Observational error of the coordinate measuring machine is calculated according to the measured data, which is stored in the storage system.