Tris(2,4-di-tert-butylphenyl)phosphite (Irgafos 168) alone and the Irgafos 168 added in the polypropylene (PP) films for food packing materials can generate degradation products under ultraviolet or microwave treatment. With n-hexane as the extraction solvent, a gas chromatography-mass spectrometry (GC-MS) method was developed for detecting Irgafos 168, and determining its degradation products, 2,4-di-tert-butylphenol and tris(2,4-di-tert-butylphenyl)phosphate, in PP food packing materials. The results showed that the calibration curves of three substances had a good linearity in the ranges of 25~150 mg/L, 0.02~20 mg/L, and 0.05~10 mg/L respectively, and the R2 values were 0.9990, 0.9989, and 0.9955, respectively. The spike recoveries at different spike concentration levels were in the range of 81.83~115.21% with RSD values in the range of 0.10~4.54%. The limits of detection (LOD) were 5.00 μg/L, 0.20 μg/L, and 25.00 μg/L, respectively, and the limits of quantitation (LOQ) were 12.00 μg/L, 0.45 μg/L, and 60.00 μg/L respectively. Therefore, this method is precise, highly sensitive, and accurate, and can be applied for detecting Irgafos 168, 2,4-di-tert-butylphenol, and tris(2,4-di-tert-butylphenyl)phosphate in PP. Irgafos 168 was added to PP, followed by ultraviolet and microwave treatments. The corresponding degradation rates were 31.88% and 11.7%, respectively, and three degradation products were generated. According to the standards, two of these degradation products were identified as 2,4-di-tert-butylphenol and tris(2,4-di-tert-butylphenyl)phosphate, and the other was speculated to be the result of Irgafos 168 with loss of the di-tert-butylphenol group. The degradation rate of Irgafos 168 standard was less than 5% under ultraviolet or microwave treatment.