To improve the measurement accuracy, the main measurement errors of the 193-nm Phase-shifting Point Diffraction Interferometer (PS/PDI) are discussed here. The elementary configuration and measuring principle of the 193-nm phase-shifting point diffraction interferometer are introduced firstly. Then different kinds of measurement errors and their causes are summed up, including reference wave front error, geometrical separation induced error, phase-shifting error, grating error, and errors caused by CCD, laser source and fluctuating surroundings and so on. The magnitude and shapes of these measurement errors, together with the connections between these errors and the configuration parameters of the interferometer, are all obtained through detailed analysis and simulation. For the sake of avoiding or restraining these errors, some methods are put forward accordingly.