The distribution of hoop stress calculated and simulated by Finite Element Method and theoretical calculation in this paper, when the hydrostatic pressure was carried out on the line pipe of Φ 762mm×31.8 mm API SPEC 5L X70 longitudinal seams submerged arc welding(LSAW). The results of simulated by Finite Element Method were accord well with that of theoretical calculation, both of them shown that the hoop stress deviation between pipe inside surface and outside surface was nearly 40MPa, which was about 9% of the hoop stress of outside surface. Besides, the influence of the D/t(diameter-thickness ratio)on the hoop stress were also researched, which shown that the larger the value of D/t was , the fewer that influence on the hoop stress, when D/t=200, the deviation was less than 1%; Or else, the D/t may cause a great stress gradient in the direction of the thickness, even when the D/t=42, the deviation was up to 5%. So that, the effect of the d/t should be taken into account when calculating the hoop stress of pipe with D/t≤42. In other word, a new hydrostatic pressure formula for the heavy wall thickness pipe( D/t≤42) was put forward and adopted when calculating its hydrostatic pressure, that was P =c* Sh (Do2 - Di2) / (Do2 + Di2), where, c, specification coefficient; Sh, hoop stress; Do, outside diameter; Di, inner diameter.