Abstract: To overcome the deficiency of inaccurate calculation on variable condition features and optimum vacuum of a condenser due to negligence of tide impact, an improvement was carried out to the calculation model by considering the influence of sea level change caused by the tide on the cooling water flow, unit incremental power and pump power consumption, on the basis of a once-through sea water cooling system in 2×800 MW power units, after which corresponding calculation results were obtained and compared with and without consideration of tide impact under variable condition features. Results show that tide has significant impact on the condenser pressure, and the impact increases with the rise of inlet water temperature of condenser. Both the optimal operation mode of pump and corresponding optimum vacuum of condenser are different calculated in the case with and without consideration of tide impact. However, the optimal operation mode of pump calculated by considering tide impact can always guarantee better economic benefits of the unit.