In order to check the validity of the International Equation of State of Seawater set up by the UNESCO in 1980 for application to China.coastal waters, the authors measured the relative density of Changjiang estuarine seawater and seawater solutions diluted with the Changjiang River water at one atmosphere .from 5-35 salinity under 15-24℃ with a high precision magnetic float densimeter designed by them. The average standard deviation of measured values was ±2.3×10-3 kgm~3. The statistical results showed that for all samples from the Changjiang estuary , the measured density with the densimeter was always higher than the density calculated by the UNESCO equation . The average differences, for dry season and flood season seawater samples were( +6.3±2.3.)×10-3kgm-3 and (+8.1±2.3)×10-3 kgm-3,respectively. The average difference between the measured and calculated (from the UNESCO equation) density increased linearly as salinity decreased and became+47.2 ×10~3kgm~3 at the Changjiang River water salinity limit of around 0.134. A linear correlation was found between this average difference and the [Ca2+]/s ratio value. The experimental average values of [Ca2+]/s, [SO42-]/s, [Mg2+]/s> and [Sr2+]/s were 0.01183±1.0×10-4,0.07698±1.7×10-4, 0.03743±8.1×10-5 and 2.26×10-4, respectively, at sample salinities of 30 to 35 . The experimental value of [Ca ]/s for the Wuhu Changjiang River water was 0.2597. The resulting densities of all samples were then fitted to an equation of the formwhere dw is the density of SMOW and B1 , B2, B3 and B4 are temperature-dependent parameters . The average standard deviation between calculated densities of this equation and experimental values was ±3.9×10-3kgm-3. In order to check the validity of the International Equation of State of Seawater set up by the UNESCO in 1980 for application to China.coastal waters, the authors measured the relative density of Changjiang estuarine seawater and seawater solutions diluted with the Changjiang River water at one The average standard deviation of measured values ​​was ± 2.3 × 10 -3 kgm ~ 3. The statistical results showed that for all samples from the Changjiang estuary, the measured density with the densimeter was always higher than the density calculated by the UNESCO equation. The average differences, for dry season and flood season seawater samples were (+ 6.3 ± 2.3.) x 10-3 kgm-3 and ( + 8.1 ± 2.3) × 10-3 kgm-3, respectively. The average difference between the measured and calculated (from the UNESCO equation) density increased linearly as salinity decreased and became + 47.2 × 10 ~ 3kgm ~ 3 at the Changjian g River water salinity limit of around 0.134. A linear correlation was found between this average difference and the [Ca2 +] / s ratio value. The experimental average values ​​of [Ca2 +] / s, [SO42-] / s, [Mg2 +] / s and Sr2 + / s were 0.01183 ± 1.0 × 10-4, 0.07698 ± 1.7 × 10-4, 0.03743 ± 8.1 × 10-5 and 2.26 × 10-4, respectively, at sample salinities of 30 to 35. The resulting densities of all samples were then fitted to an equation of the form where dw is the density of SMOW and B1, B2, B3 and B4 are temperature-dependent The average standard deviation between calculated densities of this equation and experimental values ​​was ± 3.9 × 10 -3 kgm -3.