Abstract: The sand-stone-mud padding of oil tank foundation is an inportant factor in uneven settlement of the tank basis. The paper introduces a method, called as K_\text oil testing method, for designing and inspecting the sand-stone-mud padding of large oil storage tank foundation. It explains K_30 value and its physical meaning in detail and makes a clear introduction for the determination of minimum K_30 limit required in design and for the testing method of K_30 value. The method has been proved to be accurate and reliable by a practical example.