Fourier transform infrared spectroscopy (FTIR) and pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) were applied to identify the organic materials used in the guard board mortar of Taihe Dian (Hall of Supreme Harmony) in the Forbidden City. Two guard board mortar samples, collected respectively from the upper layer and the southwest corner of the upper layer of Taihe Dian, were analyzed. FTIR results showed that in the two samples, the inorganic material used was carbonate, and that the organic materials might be lipids or shellac. Py-GC/MS analysis further confirmed that there were vegetable oils in the two samples, perhaps a mixture of raw tung oil and oil from cruciferous plants. Cedrol detected in both samples could be the volatile component of the cedar wood roof boarding of Taihe Dian. In addition, there might also be starch in the sample of upper layer guard board mortar. The present work will contribute to the scientific research and restoration of the traditional guard board mortar used in Taihe Dian.