Abstract: Since it is specified in both API Spec 5L and GB/T9711.1 - 1977 that any steel weld pipe with O.D. no smaller than 508 mm shall be subject to hydrostatic test, the hydrostatic testing machine is regarded as a rather key inspection facility. During testing, it frequently takes place that the test can not be fulfilled in one time due the unstable pressure of the tester’s main hydraulic cylinder, which not only disturbs the normal operation cycle, but also negatively influences tube quality because of repeated tests. Therefore the elements responsible for the said pressure fluctuation of the main cylinder was investigated, and then relevant modification action was taken to eliminate the influence by the booster oil control circuit on the main cylinder pressure, having satisfactory result.