The effects of complex degradation products on foaming behavior and physical properties of a 50 wt % methyldiethanolamine (MDEA) solution were investigated systematically. The degradation products in lean amine solution were detected and quantified by gas chromatographymass spectrometry (GC-MS). The degradation products were identified as N,N-dimethylethanolamine (DMEA), ethylene glycol (EG), 1-(2-hydroxyethyl)-4-methylpiperazine (HMP), diethanolamine (DEA), triethanolamine (TEA), and bicine (N,N-bis(2-hydroxyethyl)glycine), respectively. Considering the standard ASTM D892 for foam performances testing of lubricating oils, the effects of different degradation products on the foaming behavior of the MDEA solution were discussed. Results showed that the degradation compounds exhibited significant impact on foam stability promotion, even with small amounts, especially HMP and bicine. With the addition of 0.1 wt % HMP, the foam height increased from 130 mm to 325 mm; meanwhile, break time increased from 8.96 to 31.72 s. A maximum in foam height and break time was observed as the bicine was increased to 0.2 wt %. Some physical properties, particularly the surface tension, viscosity, density, and pH value of solutions had an important influence on bubble behavior.