The scheme of biomass microwave pyrolysis reactor system, gas chromatography and mass spectrometry analysis instrument, temperature adsorption analyzer, and other analysis equipment were used to study the influence of cotton stalks with NaCl, K2CO3 and MgCl2 on microwave pyrolysis characteristics. The results showed that three kinds of metal salt additives made the yield of liquid product decrease and the yield of solid product increase. The introduction of additives can simplify the bio-oil components and improve the quality effect of bio-oil obviously. Phenolic compounds were significantly lower in the product and the peak area percentage of acetic acid increased to 37.57% from 11.87%, while the surface pore structure of solid coke was also significantly improved.