Thermal decomposition behavior of NEPE propellant with CL-20 was investigated by means of TG-DTG and DSC test methods. The interactions among main ingredients NG, CL-20, AP and catalyst were discussed. The experimental results show that thermal decomposition can be divided into three stages, i.e. volatilization and decomposition of nitrate ester (NG), decomposition of PEG and CL-20, and decomposition of AP. CL-20 can accelerate decomposition of NG and PEG, while NG and PEG have little effect on decomposition of CL-20. Addition of AP accelerates decomposition of CL-20, at the same time CL-20 can make element two-step decomposition of AP merge into one-step. The interaction between Al powder and other ingredients is weak. Catalyst Ct1 and Ct2 to a certain extent inhibit initial decomposition of NG, CL-20 and AP, especially for decomposition of NG. With the temperature rising, the inhibition disappears, and thermal decomposition starts, so that decomposition reaction rate increases significantly, which can make exothermic decomposition process of the propellant shortened and burning rate increased.