(Co-) pyrolysis behaviors were quantified using TG and Py-GC/MS analyses as a function of the two fuels of sewage sludge (SS) and water hyacinth (WH), five atmospheres, six blend ratios, and three heating rates. Copyrolysis performance, gaseous characterizations and optimization analyses were conducted. Relative to N-2 atmosphere, co-pyrolysis was inhibited at low temperatures in CO2 atmosphere, while the CO2 atmosphere at high temperatures promoted the vaporization of coke. The main (co-) pyrolysis products of SS and WH were benzene and its derivatives, as well as alkenes and heterocyclic compounds. Average apparent activation energy decreased gradually with the increased atmospheric CO2 concentration and was highest (377.5 kJ/mol) in N-2 atmosphere and lowest (184.7 kJ/mol) in CO2 atmosphere. Significant interaction effects on the mean responses of mass loss, derivative TG, and differential scanning calorimetry were found for fuel type by heating rate and atmosphere type by heating rate.