The correction of initial 230Th is very important for the 230Th-234U-238U dating of impure speleothem. The corrected 230Th ages usually assume an initial 230Th/232Th atomic ratio of (4.4±2.2)×10-6. During the last 30 years, various leachleach, leachresidue methodologies have been promoted to distinguish between the isotopic ratios in the authigenic and detrital phases, and the total dissolution techniques are the most reliable method to setup meaningful isochrones. In this paper, Uranium and Thorium isotopic composition of 9 subsamples in two different layers of stalagmite MN04 have been analyzed and the respective isochrones have also been setup. The results show that the initial 230Th/232Th atomic ratio of the subsamples from up layer is (3.5±2.8)×10-6，and that of from down layer is (10.6±2.2)×10-6. It suggests that, even in the same area, the initial 230Th/232Th atomic ratio in various layers of the same stalagmite could have sizable difference because of the complicated origin of the detrital thorium. Therefore, it is insufficient to use the initial 230Th/232Th ratio obtained from the single isochron to correct the whole sample as well as other samples from the same area. Considering the high content of 232Th would also have significant influence on the measurement of 230Th, choosing pure carbonates to the best in sample selecting and preprocessing will greatly reduce the age deviation caused by the incorporated detrital thorium.