In order to accurately measure ²³²Th in the waste residues of associated radioactive mines, especially for the measurement of slag samples under non-equilibrium conditions, and to provide data support for radiation environment supervision, a method for measuring ²³²Th in slag using a high-purity germanium γ spectrometer was established. The method involves indirect measurement by selecting the characteristic γ rays of the daughter nuclides in the decay chain of ²³²Th, and calculating its specific activity by the relative comparison method. The slag samples in a non-equilibrium state are sealed for 20 days to enable ²²⁸Ra and ²²⁸Ac to reach equilibrium, and ²²⁸Th, ²²⁴Ra and subsequent daughter nuclides to reach equilibrium. Then, the specific activity of the parent nuclide ²³²Th is obtained through conversion relationships. This method is accurate and efficient, with a detection limit of 1.1 Bq・kg^-1, a precision better than 5 %, and a relative error lower than 5 %. The method of measuring ²³²Th in slag using a high-purity germanium γ spectrometer does not require complex chemical treatment, which makes up for the deficiencies of inductively coupled plasma mass spectrometry. The calculation method under non-equilibrium conditions is also more reliable than using the average value of a single characteristic peak or multiple characteristic peaks.