Phase shifting interferometry is commonly used in precision optical surface measurement, but which possesses some limitation because of the sensitivity to environment. Therefore, it is hardly used in optical testing in the workshop environment. Thus, the instantaneous interferometry is a good choice because of the insensitive to vibration. This paper will describe an instantaneous interferometry utilizing spatial carrier and Fourier transform, and discuss the accuracy of the interferometer for optical testing when phase-shifting interferometry is unable to realize the precision measurement. With a lot of experiments, some problems were analyzed, including the relationships between the measurement accuracy and systematic error, vibration, temperature, the test surface cleanliness and so on. The discussed work of error restraint can provide a reference for the instantaneous interferometry applications.