Co-pyrolysis corn stalk (CS) and high-density polyethylene (HDPE) is an effective method to mitigate environmental pollution and reuse the problematic plastic to produce valuable energy. The devolatilization kinetics and the evolution of gaseous species during co-pyrolysis were investigated via TG-FTIR-MS. Moreover, the possible interactions of CS and HDPE were explored. The co-pyrolysis process of CS-HDPE blends was found to be divided into two decomposition stages. The first stage (150-400 degrees C) was mainly associated with the decomposition of CS. The second stage (400-515 degrees C) was the result of combinations of the thermal degradations of CS and HDPE, with the degradation of HDPE contributing the most. Co-pyrolysis CS with HDPE could delay the decomposition of HDPE. In addition, the difference between experimental and theoretical weight loss (Delta W) was found to be less than zero when the percentage of CS was 80%. Furthermore, the average of the experimental activation energy was lower than that of the theoretical value for 80% CS. Thus, the positive synergy was strongest when the CS content was 80%, based on the thermal decomposition behavior and the activation energy. However, the blending ratio of CS-HDPE had little effect on the major categories of gaseous products during co-pyrolysis. Moreover, the interaction between CS and HDPE in the first stage (150-400 degrees C) could promote the release of H-2, CO/C2H4 and C3H6 in the blends with 80%, 60%, and 60% CS, respectively. Additionally, aliphatic hydrocarbons (CH4, C2H6, C3H8, C4H10, C4H8, C2H2) and oxygen-containing compounds (aldehydes, alcohols, ketones, acids) were suppressed by the interaction when the percentage of CS was less than 60%. In the second stage (400-850 degrees C), the yields of aliphatic hydrocarbons (CH4, C2H6, C3H8, C4H10, C4H8, C2H2, C4H6) were inhibited, independent of the content of CS in the blends. In addition, the production of H-2, CO/C2H4, and C3H6 was promoted when the percentage of CS was below 80%, 80%, and 60%, respectively.