A two-step catalytic co-pyrolysis (TSCCP) of walnut shell (WNS) and low-density polyethylene (LDPE) was innovatively studied at a lab-scale fixed-bed reactor using HZSM-5 as a catalyst. Various characterization techniques such as FTIR, Raman spectroscopy, SEM, thermal gravimetric analysis (TGA) and Van Krevelen diagram were applied to explore structure evolutions of the derived chars aiming at revealing the step-wise copyrolysis mechanisms. In comparison with conventional one-step catalytic co-pyrolysis (OSCCP), the TSCCP has much higher oil production, and less gas and solid yields. The yield of aromatics increased by 34.2 %. Oxygenated compounds in oil were dramatically reduced. As a result, more water was generated. These experimental results have demonstrated that the two-step approach can significantly enhance the synergy of WNS with LDPE. Characterizations revealed that the TSCCP stages the interactions of cellulose, hemicellulose and lignin with LDPE. The interaction of lignin with LDPE was thoroughly examined. Possible mechanisms were put forward to explain the synergistic effects of WNS with LDPE enhanced by the two-step approach.