Dow proposed a method by using the differential of vitrinite reflectance of the upper and lower strata to estimate the eroded strata thickness at an unconformity surface in 1977. This method is still widely used in China. But some Chinese scholars have already recognized the irrationality of the method and then have proposed some revising methods. Among them, the “method of the highest paleotemperature(MHP)” is representative due to its rational idea. In order to estimate the eroded strata thickness at an unconformity surface by using vitrinite reflectance data more reasonably and more conveniently, the author has improved the MHP in this paper and got a new method. The improved new method not only inherited the rational idea of MHP but also much simplified calculation process. There are some premises in estimating the eroded strata thickness of the top surface of a structural layer by using Ro data. If all the premises are established, we can directly project the linear regressive relationship between ln (Ro) and H of this structural layer upward to the value of ln (0.2) and get the approximate paleosurface’s position. The distance between paleosurface and unconformity surface is the expected erodedstrata thickness. Recalculating the Dow’s original data with MHP and the new method, we can get the results of 2735 m and 2537 m respectively. These two close results show the feasibility of the new method. And compared with MHP, the new method is more convenient. Both of the close results are far away from Dow’s result of 500 m. Then the author discusses the irrationality of Dow’s result and principle from several aspects. The new method will be widely applied in many branch disciplines of geology.