In this paper,we analyze the error of water terahertz (THz) optical parameters u sing terahertz vertical reflection time-domain spectroscopy.First,the refractive index and th e absorption coefficient are extracted by the transform function of the ratio of the referenc e signal from air-silicon interface to the sample signal from silicon-water interface on the high resist ance silicon wafer,and the errors of these optical parameters are analyzed by error propagation and com bination theory. The results show that the random errors of the refractive index and the absorpti on coefficient from terahertz time-domain spectroscopy (THz-TDS) multi-measurement almost keep unchanged between 0.1－1.1THz,but the y gradually become bigger when reaching 0.1THz and 1.1THz as U like distribution.It indica tes the sensitivity of the instrument decreases near 0.1THz and 1.1THz.The errors from the thickness and refractive index are much bigger than those from THz-TDS.For the refractive index of water, the errors keep little changed,while for absorption,the errors are increasing w it h frequency increasing.The method can be applied to the THz optical parameters extraction a nd error analysis in similar terahertz vertical reflection time domain spectroscopy.