In order to solve the problem of structural damage or seal leakage caused by improper installation preload of ultra-high pressure fracturing manifold union,the 175 MPa ultra-high pressure union developed by a company was taken as the research object,and the contact model was established by ABAQUS.The stress analysis,strength evaluation and sealing performance judgment of the union were carried out by using the finite element method.The analysis results show that the preload has almost no effect on the stress of the female union,but has a significant effect on the stress of the male union,the airfoil nut and the three-petal retaining ring,and all of them show a positive correlation.When the preload exceeds 250 kN under the rated working conditions,the stress at the fillet of the shoulder of the male union exceeds the allowable stress of the material,and the structural static strength does not meet the requirements and may cause structural damage.When the preload is less than 150 kN,it is not enough to resist the internal pressure,and the male and female ball cone sealing structures are separated.Within the preload range of 150 kN to 250 kN that can ensure the tightness of the ball cone and the structural safety,the surface contact stress of the HNBR sealing ring is always greater than the real-time fluid working pressure,and the sealing ring has good sealing performance.The recommended installation preload range is 150 kN to 250 kN,within which the sealing ring structure can meet the requirements of static strength and sealing performance.