The main objective of the research is to combine a method of spectrophotometry with the technique of solid-phase extraction to enrich mercury (Hg) for determination of trace amount of Hg in water. The trace amount of mercury is measured by the spectrophotometric method using dithizone as colour forming reagent in the micelle environment of Sodium dodecyl sulfate (SDS). Factors which can affect the measurment such as pH, micelle environment, dithizone concentration. effects of other metals, masking compounds were investigated. The calibration curve was instructed within the linear range from 0.1 ppm to 3.0 ppm. This method has high sensitivity and good repeatability, which satisfies analysis requirements. To enrich the amount of Hg before the measurement, a home-made solid-phase material - known as modified rice husk which is synthesized from agricultural waste was used. The result showed that this method is applicable to determine the amount of Hg with the concentration of ppb in natural water samples from West Lake water, Hoan Kiem Lake and so on. Besides, the error of this method is insignificant compared to that of the standard method - CV-AAS.