HFO-1336mzz(Z) is a promising working fluid used in high-temperature heat pump and organic Rankine cycle (ORC) system because of its environmental friendly features and good thermal performance. In this work, a test system is designed to assess the thermal stability of HFO-1336mzz(Z). The fluoride ion concentration is used as an indicator of HFO-1336mzz(Z) dissociation. The experimental results show that the pressure has a great effect on the dissociation of HFO-1336mzz(Z) and the dissociation temperatures of HFO-1336mzz(Z) at 2.1, 3.1, and 4.0 MPa for 24 hours are 310 degrees C to 330 degrees C, 290 degrees C to 310 degrees C, and 270 degrees C to 290 degrees C, respectively. The decomposition products of HFO-1336mzz(Z) are measured by a Fourier transform infrared spectrometer (FTIR); the main decomposition products are HF, CF4, CHF3, C2F(4), C2F6, C2HF5, and C3F8. Finally, density functional theory (DFT) method is used to study the decomposition mechanism of HFO-1336mzz(Z). The main initial decomposition reaction is the fracture of C(sic)C bond into C-C bond, and then the CF3 group is separated from HFO-1336mzz(Z) molecule to produce CF3 radical. H, F-abstraction, and combination reactions are important in the consequent reactions to generate the main decomposition products.