Abstract: Based on the physical nonlinearity of soil, geometrical nonlinearity and longitudinal and horizontal buckling of pipe beam, some equations in calculating the longitudinal stability for initial buckling of buried pipelines have been derived, some of the equations which are slightly reduced are the equations recommended from the CECS 15:90 《Specification of Buried Oil & Gas Steel Pipeline Structure Design》. The availability of the equations is disscussed in the paper. Because the physical nonlinearity of soil is on the basis of experiment, its mathematical and mechanical deduction are rigrous, provided that the reduction is reasonable, the equations listed in the paper will be practical as well as generally simply safe, which have been proved by experiments and engineering. The study on the longitudinal stability for buried pipelines belongs under the category of large displacement nonlinear mechanics, the analytical solution is generally difficult to obtain, so that the serial analytical solutions derived from the paper are simple and practical.