Because of its special construction technique, the power drive of the shield machine is basically supported by hydraulic system, and therefore, the stable operation of the hydraulic system will directly affect the construction efficiency of the shield machine. Based on the past construction experience, the failure of the hydraulic system is caused by non-purified hydraulic oil to some extent, so the improvement of the purity level of the hydraulic oil is critical to ensure the stable operation of the hydraulic system. From the perspective of the improvement of the purity of the hydraulic oil, a centrifugal oil purifier is introduced and by-pass circulation purification is carried out at the most critical place where the oil tank of the shield machine is. After 2-month purification, remarkable effects that the hydraulic system keeps stable, and the consumption of hydraulic oil and filter element has been greatly reduced have gained, so that this technological modification can be used as a reference to the modification of equipment like the hydraulic system of the shield machine.