It is a widely consensus that the ambient temperature direct water injection can be utilized to reduce nitric oxides emissions in both diesel and gasoline engine. Since the proposal of water injection technology, there are many researches studying the effect of different direct water injection strategies on nitric oxides emissions and engine performance which generally lead to lower nitric oxides emissions while combustion efficiency deteriorated. The present work aimed at optimizing diesel engine combustion efficiency with the decrement of nitric oxides emissions using optimized water injection strategy. The engine has been modified based on a two cylinder mechanical pump diesel engine into common rail diesel engine with capability in direct water injection. The direct water injection system is designed and manufactured independently, an air-fluid booster is utilized to establish the water injection pressure up to 40MPa. Customized diesel injector is selected to be used as water injector in this study. Water injection strategy is optimized in detail with injection timing around TDC which ranges from 12°CA BTDC to 5°CA ATDC under 17 bar IMEP. The engine efficiency can be improved under selected water injection strategy due to the increment of work fluid in the combustion chamber. Moreover, The nitric oxides emissions show decrement around 10% to 15%. As the water injection timing approaching TDC, the efficiency enhancement rate increases and reaches its peak value at 5°CA BTDC. The enhancement rate drops as the injection timing further postpones. The water injection timing shows little effect on the nitric oxides emissions under the selected water injection duration. It can be predicted that the enhancement rate can be improved as engine load increases.