Abstract: The current hydraulic impact technologies face various problems such as poor adaptability of associated tools in limited well conditions with unstable performance, and the mechanism of the technologies remains untested or verified. Therefore, the hydraulic impact tools were optimized and tested downhole according to the reservoir and engineering characteristics of offshore oilfields. A hydraulic impact tool customized to offshore oilfields was developed by optimization of its size, material and compressive strength, and modification of the key components such as the impact plates. Downhole tests of the hydraulic impact tool were carried out in three wells with low productivity and efficiency in Changqing Oilfield, the formation condition of which was closed to that of offshore oilfields. In the tests, 6–20 pulse wave forms lasting 20–60 s were collected, the main mechanism of the technology was the hydraulic pulse. The test results demonstrated that the optimized tool achieved stable performance and the modified impact plates exhibited higher precision of fracturing pressure as well. These technology significantly enhanced the injectivity of the water injectors. The optimized hydraulic impact tool provides a basis for the application of the technology in offshore oilfields with large wellbores.