To determine the thorium capture ratio and thorium-uranium conversion ratio in the thorium cladding of the fusion-fission reactor model, a new approach, activated thorium sample decay γ -ray off-line measurement, is introduced in this paper. This method is based on the measurement of the 311.98 keV characteristic decay γ-ray emitted by 233Pa which is produced through decay of 233Th, the capture product of 232Th. Then some reverse mathematic approach is brought in and finally the 232Th (n, γ ) 233Th reaction ratio is determined, with an uncertainty of 6% in the test experiment (233Th/232Th under the order of magnitude 10-17). In the paper, we introduce the detail of the approach including the background, principle and the result of a simple test experiment which shows that this approach can work well in thorium capture ratio determination. Compared with prompt γ-ray method and mass spectrometer method, this decay γ-ray off-line method has many advantages in determining the thorium capture ratio and thorium-uranium conversion ratio in the thorium cladding. It is also promising in the measurement of other relevant parameters.