This paper integrates three methods to constraint a time span from Late Mesozoic to Cenozoic when the lithospheric thinning took place in the eastern North China Block. These methods include comparison of equilibrium P-T conditions of mantle-derived xenoliths with different ages, reflection of the lithospheric thickness by the geochemical characteristics of basalt and comparison of the geochemical characteristics of lithospheric mantle with different ages. The mechanism of thinning may be the spread process of continental lithosphere after Late Mesozoic in this region. The lithospheric thickness was ultimately thinned to less than 70 km by mechanical spread as well as thermal-mechanical and chemical erosion. However, the old lithospheric mantle did not whole disappear after thinning. The rudimental part of it was intensively altered by the materials derived from the asthenosphere, making its Sr-Nd isotopic composition similar to the asthenosphere, while its Os isotopic composition was not evidently influenced. This altered lithospheric mantle constituted the main body of Cenozoic lithospheric mantle in the eastern North China Block. The lithospheric thinning presented non-uniform characteristics in space and time.