On the basis of analyzing the nature and feature of hard-water effect, the hard-water effect calibration of sediment ¹⁴C ages of Lake Qinghai is carried out using the relationship between ¹⁴C levels in lake waters and the atmosphere , in the light of marine reservoir correction. The variation of the hard water effect in Lake Qinghai back to 8000 aBP is also reconstructed. It’s concluded that ¹⁴C ages of the surface sediment in Lake Qinghai is less than 1549 aBP and the hard-water effect of Lake Qinghai increased evidently during 7000-1000aBP.