A 32cm long sediment-core was sampled from Kechou reservoir located in Karst area of central Guizhou Province. According to study on grain size pattern and ¹³⁷ Cs dating in the profile, comparing with precipitation records, we determined the sediment ages in different depth of this core. Using the cumulative mass depth, we also calculated the deposit rates in different stages from 1960 to 2004. We improved the traditional ¹³⁷ Cs dating method which only relied on the position of peaks in profile. By analyzing the ¹³⁷ Cs depositing transportation and sedimentation, we found that the absolute values of ¹³⁷ Cs in the certain small catchment often appear in the low level. We believe that the soil erosion, which was caused by precipitation and other reasons, should intensively affect the amount of ¹³⁷ Cs depositing from soil to the sediment in reservoir. That should be the main reason of different pattern of ¹³⁷ Cs distributions in the sediment-core.