Abstract: In order to ensure the scientific and accurate calculation of oil and gas up-channeling velocity and solve the problem of inaccurate or even negative calculation results in the lag time method, a relative time method with more accurate calculation results has been proposed. However, check valves have been widely used in drilling, and the drilling fluid during drilling all return from the annular space without entering the drilling tool, making the calculation result of the relative time method no longer accurate. In order to adapt to the new drilling technology, the original calculation model and formula of the relative time method should be modified. Therefore, calculation formula for drilling bit depth under the bottom of the oil and gas reservoir were added, and a new series of calculation formula for three kinds of casing programs and twelve kinds of drilling depth were provided. The real cases showed that the reliability and applicability of the revised calculation formula were further improved.