Thermal conversion is a promising approach for the disposal of phytoremediation biowastes. The thermal characteristics of potherb mustard biowaste contaminated by potentially toxic metals were studied in this work. Thermogravimetric (TG) and derivative thermogravimetric (DTG) analyses of four feedstocks with different Cd/Zn content were conducted in an inert environment. Evolved gases were identified online by mass spectrometry (MS) coupled with TG. The TG-DTG profiles of the four feedstocks presented similar decomposition stages, whereas the samples with low Cd/Zn had a faster degradation rate, suggesting that the presence of Cd/Zn had little effect on thermal decomposition mechanism. The maximum evolution of H and CH occurred at 500-650 °C, while CO was released at a low temperature. PTMs could catalyze the H production during the pyrolysis of biowaste. This investigation revealed the effects of different Cd/Zn contents on thermal conversion, providing a practical reference for the thermal treatment of phytoremediation biowastes.