Abstract: It has been a dispute for a long time how to calculate the critical outer pressure on non-uniform wall casing,since it is difficult to get out the solution from the binomial differential equation established when casing is deformed.The previous researchers derivated the formula of the critical outer pressure by methods of energy and variation rather than from the equation.Ihey were used with supposing the deformation function of the bending of uniform casing wall,and resulted in error.The paper analyzes the existing problems and derivates the approximate formula of critical outer pressure of non-uniform wall casing,converting differential equation with variabl coefficient to normal one,through some point values in the model of mechanics.The paper says that the new formula is in keeping with the actual field result,based on case calculation.