The characteristics of stress distribution in high-pressure vessel are analyzed at first. Then the formula of inner pressure is derived. Through analysis the formula, a conclusion that the maximal inner pressure the high-pressure vessel can support goes steady under the diameter ratio is more than 5. Then the formula of equivalent stress at the dangerous position (inside wall) in high-pressure vessel is educed according to the fourth strength theorem and the quantitative relationship in the vessel’s inside pressure and diametrical ratio and the fatigue limit further is founded. Finally, the advice that proper structure is adopted to improve the cylinder’s inner pressure when design thick wall high-pressure cylinder according to relationships as above. These advices have a definite directive significance for how to design and use vessel effectively.