  Measuring the quasar distance through joint analysis of spectroastrometry
(SA) and reverberation mapping (RM) observations is a new method for driving
the development of cosmology. In this paper, we carry out detailed simulation
and analysis to study the effect of four basic observational parameters
(baseline length, exposure time, equivalent diameter and spectral resolution)
on the data quality of differential phase curves (DPCs), furthermore on the
accuracy of distance measurement. In our simulation, we adopt an axis
symmetrical disc model of broad line region (BLR) to generate differential
phase signals. We find that the differential phases and their Poisson errors
could be amplified by extending the baseline, while the influence of OPD
(optical path difference) errors can be reduced during fitting the BLR model.
Longer exposure time or larger equivalent diameter helps reduce the absolute
Poisson error. Therefore, the relative error of DPCs could be reduce by
increasing any of the above three parameters, then the accuracy of distance
measurement could be improved. In contrast, the uncertainty of $D_{\rm{A}}$ (
absolute angular distances) could be improved with higher spectral resolution,
although the relative error of DPCs would be amplified. We show how the
uncertainty of distance measurement varies with the relative error of DPCs. For
our specific set of model parameters, without considering more complicated
structures and kinematics of BLRs in our simulation, it is found that the
relative error of DPCs $<$ 20$\%$ is a limit for accurate distance measurement.
The relative error of DPCs have a lower limit (roughly 5$\%$) and the
uncertainty of distance measurement can be better than 2$\%$.
