The Chinese Survey Space Telescope (CSST) is the first large space optical telescope of China. It will perform scientific observations and yield important scientific results, e.g. multi-color photometry to find and characterize exoplanets. The precision of time-domain photometry affected by various astrophysical and instrument noises is crucial for exoplanet detection and characterization. This paper constructs a time-domain model to simulate the stellar signal and various noises based on the main technical parameters of CSST published so far. The precision of photometry is analyzed by modeling the signal and noises for an example of the gazing mode in the i band. We investigate and discuss the contribution of various noises while doing aperture photometry, especially the jitter noise caused by pointing jitter and pixel response inhomogeneity. The test results also provide the recommended size of aperture photometry. Jitter noise can be suppressed by reducing jitter amplitude and uniforming pixel or by using differential photometry with reference stars. Our simulation results can be applied to the precision prediction of time-domain photometry, the refinement of CSST instrumental specifications, evaluation of exoplanet detection capabilities, and data reduction progress for CSST.